diff --git a/.idea/caches/build_file_checksums.ser b/.idea/caches/build_file_checksums.ser
index 33a95c0..1b56c5d 100644
Binary files a/.idea/caches/build_file_checksums.ser and b/.idea/caches/build_file_checksums.ser differ
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/MainContract.java b/app/src/main/java/ru/volgorobot/vrcatalog/MainContract.java
index cfabe87..2e7232f 100644
--- a/app/src/main/java/ru/volgorobot/vrcatalog/MainContract.java
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/MainContract.java
@@ -1,11 +1,8 @@
 package ru.volgorobot.vrcatalog;
 
-import android.widget.ImageView;
-
-import com.github.msteinbeck.sig4j.signal.Signal1;
-
 import java.util.ArrayList;
 
+import ru.volgorobot.vrcatalog.additional.NetworkErrorException;
 import ru.volgorobot.vrcatalog.model.FirstLevelModel;
 import ru.volgorobot.vrcatalog.model.SecondLevelModel;
 import ru.volgorobot.vrcatalog.model.ThirdLevelModel;
@@ -20,13 +17,13 @@ public interface MainContract {
 
     interface Presenter {
         void getFirstLevel();
-        void failureAnswerSlot(int errorCode);
     }
 
     interface MainModel {
-        Signal1<Integer> errorSignal = new Signal1<>();
-        ArrayList<FirstLevelModel> getFirstLevel();
-        ArrayList<SecondLevelModel> getSecondLevelByParentID(int parentID);
-        ArrayList<ThirdLevelModel> getThirdLevelByParentID(int parentID);
+        ArrayList<FirstLevelModel> getFirstLevel() throws NetworkErrorException, NullPointerException;
+        ArrayList<SecondLevelModel> getSecondLevelByParentID(int parentID) throws NetworkErrorException, NullPointerException;
+        ArrayList<ThirdLevelModel> getThirdLevelByParentID(int parentID) throws NetworkErrorException, NullPointerException;
+        void reinitializeApi() throws IllegalStateException;
+        boolean getApiState();
     }
 }
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/MainPresenter.java b/app/src/main/java/ru/volgorobot/vrcatalog/MainPresenter.java
index eba079a..c707bef 100644
--- a/app/src/main/java/ru/volgorobot/vrcatalog/MainPresenter.java
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/MainPresenter.java
@@ -5,6 +5,8 @@ import android.os.AsyncTask;
 
 import java.util.ArrayList;
 
+import ru.volgorobot.vrcatalog.additional.NetworkErrorException;
+import ru.volgorobot.vrcatalog.additional.ResultWithErrorCode;
 import ru.volgorobot.vrcatalog.model.CoreModel;
 import ru.volgorobot.vrcatalog.model.FirstLevelModel;
 
@@ -14,46 +16,53 @@ public class MainPresenter implements MainContract.Presenter {
 
     public MainPresenter(MainContract.MainActivityView mainActivityView, Context context) {
         this.mView = mainActivityView;
-        try {
-            this.coreModel = new CoreModel(context);
-        } catch (IllegalStateException e) {
-            mView.onFailureAnswer(3);
-        }
-        coreModel.errorSignal.connect(this::failureAnswerSlot);
+        this.coreModel = new CoreModel(context);
     }
 
     @Override
     public void getFirstLevel() {
-        new AsyncTask<Void, Void, ArrayList<FirstLevelModel>>() {
+        try {
+            coreModel.reinitializeApi();
+        } catch (IllegalStateException e) {
+            mView.onFailureAnswer(3);
+            return;
+        }
+        new AsyncTask<Void, Void, ResultWithErrorCode<ArrayList<FirstLevelModel>>>() {
             @Override
-            protected ArrayList<FirstLevelModel> doInBackground(Void... voids) {
-                ArrayList<FirstLevelModel> firstLevelModels = coreModel.getFirstLevel();
-                return firstLevelModels;
+            protected ResultWithErrorCode<ArrayList<FirstLevelModel>> doInBackground(Void... voids) {
+                ArrayList<FirstLevelModel> firstLevelModels = null;
+                try {
+                    firstLevelModels = coreModel.getFirstLevel();
+                } catch (NetworkErrorException networkErrorException) {
+                    return new ResultWithErrorCode<>(firstLevelModels, networkErrorException.getErrorCode());
+                } catch (NullPointerException nullPointerException) {
+                    return new ResultWithErrorCode<>(firstLevelModels, 3);
+                }
+                return new ResultWithErrorCode<>(firstLevelModels, 0);
             }
 
             @Override
-            protected void onPostExecute(ArrayList<FirstLevelModel> firstLevelModels) {
-                mView.buildFirstLevelTree(firstLevelModels);
+            protected void onPostExecute(ResultWithErrorCode<ArrayList<FirstLevelModel>> result) {
+                switch (result.getErrorCode()) {
+                    case 0: {
+                        mView.buildFirstLevelTree(result.getData());
+                        break;
+                    }
+                    case 1: {
+                        mView.onFailureAnswer(1);
+                        break;
+                    }
+                    case 2: {
+                        mView.onFailureAnswer(2);
+                        break;
+                    }
+                    case 3: {
+                        mView.onFailureAnswer(3);
+                        break;
+                    }
+                }
+                mView.swipeLayoutSetRefreshing(false);
             }
         }.execute();
-        mView.swipeLayoutSetRefreshing(false);
-    }
-
-    @Override
-    public void failureAnswerSlot(int errorCode) {
-        switch (errorCode) {
-            case 1: {
-                mView.onFailureAnswer(1);
-                break;
-            }
-            case 2: {
-                mView.onFailureAnswer(2);
-                break;
-            }
-            case 3: {
-                mView.onFailureAnswer(3);
-                break;
-            }
-        }
     }
 }
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/additional/NetworkErrorException.java b/app/src/main/java/ru/volgorobot/vrcatalog/additional/NetworkErrorException.java
new file mode 100644
index 0000000..ea9bf16
--- /dev/null
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/additional/NetworkErrorException.java
@@ -0,0 +1,13 @@
+package ru.volgorobot.vrcatalog.additional;
+
+public class NetworkErrorException extends Exception {
+    private int errorCode;
+
+    public NetworkErrorException(int errorCode) {
+        this.errorCode = errorCode;
+    }
+
+    public int getErrorCode() {
+        return errorCode;
+    }
+}
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/additional/ResultWithErrorCode.java b/app/src/main/java/ru/volgorobot/vrcatalog/additional/ResultWithErrorCode.java
new file mode 100644
index 0000000..6d20600
--- /dev/null
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/additional/ResultWithErrorCode.java
@@ -0,0 +1,28 @@
+package ru.volgorobot.vrcatalog.additional;
+
+
+public class ResultWithErrorCode<T> {
+    private T data;
+    private int errorCode;
+
+    public ResultWithErrorCode(T data, int errorCode) {
+        this.data = data;
+        this.errorCode = errorCode;
+    }
+
+    public T getData() {
+        return data;
+    }
+
+    public int getErrorCode() {
+        return errorCode;
+    }
+
+    public void setData(T data) {
+        this.data = data;
+    }
+
+    public void setErrorCode(int errorCode) {
+        this.errorCode = errorCode;
+    }
+}
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/model/CoreModel.java b/app/src/main/java/ru/volgorobot/vrcatalog/model/CoreModel.java
index 8f24604..90e2498 100644
--- a/app/src/main/java/ru/volgorobot/vrcatalog/model/CoreModel.java
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/model/CoreModel.java
@@ -11,18 +11,88 @@ import java.util.List;
 
 import retrofit2.Response;
 import ru.volgorobot.vrcatalog.MainContract;
+import ru.volgorobot.vrcatalog.additional.NetworkErrorException;
 import ru.volgorobot.vrcatalog.api.Controller;
 import ru.volgorobot.vrcatalog.api.VRApi;
 
 public class CoreModel implements MainContract.MainModel {
     private Context context;
     private VRApi vrApi;
+    private String baseURL;
     public final Signal1<Integer> errorSignal = new Signal1<>();
 
-    public CoreModel(Context context) throws IllegalStateException {
+    public CoreModel(Context context) {
         this.context = context;
         SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(context);
-        String baseURL = sharedPreferences.getString("addressOfServer", "");
+        this.baseURL = sharedPreferences.getString("addressOfServer", "");
+        try {
+            vrApi = Controller.getApi(baseURL);
+        } catch(IllegalStateException e) {
+           // Nothing
+        }
+    }
+
+    @Override
+    public ArrayList<FirstLevelModel> getFirstLevel() throws NetworkErrorException, NullPointerException {
+        if(vrApi == null) {
+            throw new NullPointerException();
+        }
+        final ArrayList<FirstLevelModel> firstLevelModels = new ArrayList<>();
+        try {
+            Response<List<FirstLevelModel>> response = vrApi.getFirstLevel().execute();
+            if(response.isSuccessful()) {
+                firstLevelModels.addAll(response.body());
+            } else {
+                throw new NetworkErrorException(2);
+            }
+        } catch (IOException e) {
+            throw new NetworkErrorException(1);
+        }
+        return firstLevelModels;
+    }
+
+    @Override
+    public ArrayList<SecondLevelModel> getSecondLevelByParentID(int parentID) throws NetworkErrorException, NullPointerException {
+        if(vrApi == null) {
+            throw new NullPointerException();
+        }
+        final ArrayList<SecondLevelModel> secondLevelModels = new ArrayList<>();
+        try {
+            Response<List<SecondLevelModel>> response = vrApi.getSecondLevel(parentID).execute();
+            if(response.isSuccessful()) {
+                secondLevelModels.addAll(response.body());
+            } else {
+                throw new NetworkErrorException(2);
+            }
+        } catch (IOException e) {
+            throw new NetworkErrorException(1);
+        }
+        return secondLevelModels;
+    }
+
+    @Override
+    public ArrayList<ThirdLevelModel> getThirdLevelByParentID(int parentID) throws NetworkErrorException, NullPointerException {
+        if(vrApi == null) {
+            throw new NullPointerException();
+        }
+        final ArrayList<ThirdLevelModel> thirdLevelModels = new ArrayList<>();
+        try {
+            Response<List<ThirdLevelModel>> response = vrApi.getDetails(parentID).execute();
+            if(response.isSuccessful()) {
+                thirdLevelModels.addAll(response.body());
+            } else {
+                throw new NetworkErrorException(2);
+            }
+        } catch (IOException e) {
+            throw new NetworkErrorException(1);
+        }
+        return thirdLevelModels;
+    }
+
+    @Override
+    public void reinitializeApi() throws IllegalStateException {
+        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(context);
+        this.baseURL = sharedPreferences.getString("addressOfServer", "");
         try {
             vrApi = Controller.getApi(baseURL);
         } catch(IllegalStateException e) {
@@ -31,50 +101,11 @@ public class CoreModel implements MainContract.MainModel {
     }
 
     @Override
-    public ArrayList<FirstLevelModel> getFirstLevel() {
-        final ArrayList<FirstLevelModel> firstLevelModels = new ArrayList<>();
-        try {
-            Response<List<FirstLevelModel>> response = vrApi.getFirstLevel().execute();
-            if(response.isSuccessful()) {
-                firstLevelModels.addAll(response.body());
-            } else {
-                errorSignal.emit(2);
-            }
-        } catch (IOException e) {
-            errorSignal.emit(1);
+    public boolean getApiState() {
+        if(vrApi != null) {
+            return true;
+        } else {
+            return false;
         }
-        return firstLevelModels;
-    }
-
-    @Override
-    public ArrayList<SecondLevelModel> getSecondLevelByParentID(int parentID) {
-        final ArrayList<SecondLevelModel> secondLevelModels = new ArrayList<>();
-        try {
-            Response<List<SecondLevelModel>> response = vrApi.getSecondLevel(parentID).execute();
-            if(response.isSuccessful()) {
-                secondLevelModels.addAll(response.body());
-            } else {
-                errorSignal.emit(2);
-            }
-        } catch (IOException e) {
-            errorSignal.emit(1);
-        }
-        return secondLevelModels;
-    }
-
-    @Override
-    public ArrayList<ThirdLevelModel> getThirdLevelByParentID(int parentID) {
-        final ArrayList<ThirdLevelModel> thirdLevelModels = new ArrayList<>();
-        try {
-            Response<List<ThirdLevelModel>> response = vrApi.getDetails(parentID).execute();
-            if(response.isSuccessful()) {
-                thirdLevelModels.addAll(response.body());
-            } else {
-                errorSignal.emit(2);
-            }
-        } catch (IOException e) {
-            errorSignal.emit(1);
-        }
-        return thirdLevelModels;
     }
 }
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/FirstLevelNodeViewBinder.java b/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/FirstLevelNodeViewBinder.java
index c08768f..52865a2 100644
--- a/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/FirstLevelNodeViewBinder.java
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/FirstLevelNodeViewBinder.java
@@ -12,6 +12,8 @@ import me.texy.treeview.TreeNode;
 import me.texy.treeview.base.BaseNodeViewBinder;
 import ru.volgorobot.vrcatalog.MainContract;
 import ru.volgorobot.vrcatalog.R;
+import ru.volgorobot.vrcatalog.additional.NetworkErrorException;
+import ru.volgorobot.vrcatalog.additional.ResultWithErrorCode;
 import ru.volgorobot.vrcatalog.model.CoreModel;
 import ru.volgorobot.vrcatalog.model.FirstLevelModel;
 import ru.volgorobot.vrcatalog.model.SecondLevelModel;
@@ -45,7 +47,7 @@ public class FirstLevelNodeViewBinder extends BaseNodeViewBinder {
     public void onNodeToggled(TreeNode treeNode, boolean expand) {
         if(expand) {
             FirstLevelModel firstLevelModel = (FirstLevelModel) treeNode.getValue();
-            new AsyncTask<Void, Void, ArrayList<SecondLevelModel>>() {
+            new AsyncTask<Void, Void, ResultWithErrorCode<ArrayList<SecondLevelModel>>>() {
                 @Override
                 protected void onPreExecute() {
                     super.onPreExecute();
@@ -53,24 +55,49 @@ public class FirstLevelNodeViewBinder extends BaseNodeViewBinder {
                 }
 
                 @Override
-                protected ArrayList<SecondLevelModel> doInBackground(Void... voids) {
-                    ArrayList<SecondLevelModel> secondLevelModels = coreModel.getSecondLevelByParentID(firstLevelModel.getID());
-                    return secondLevelModels;
+                protected ResultWithErrorCode<ArrayList<SecondLevelModel>> doInBackground(Void... voids) {
+                    ArrayList<SecondLevelModel> secondLevelModels = null;
+                    try {
+                        secondLevelModels = coreModel.getSecondLevelByParentID(firstLevelModel.getID());
+                    } catch (NetworkErrorException networkErrorException) {
+                        return new ResultWithErrorCode<>(secondLevelModels, networkErrorException.getErrorCode());
+                    } catch (NullPointerException nullPointerException) {
+                        return new ResultWithErrorCode<>(secondLevelModels, 3);
+                    }
+
+                    return new ResultWithErrorCode<>(secondLevelModels, 0);
                 }
 
                 @Override
-                protected void onPostExecute(ArrayList<SecondLevelModel> secondLevelModels) {
-                    super.onPostExecute(secondLevelModels);
-                    ArrayList<TreeNode> treeNodes = new ArrayList<>();
-                    for (int i = 0; i < secondLevelModels.size(); i++) {
-                        TreeNode treeNode1 = new TreeNode(secondLevelModels.get(i));
-                        treeNode1.setLevel(1);
-                        treeNodes.add(treeNode1);
+                protected void onPostExecute(ResultWithErrorCode<ArrayList<SecondLevelModel>> result) {
+                    super.onPostExecute(result);
+                    switch (result.getErrorCode()) {
+                        case 0: {
+                            ArrayList<TreeNode> treeNodes = new ArrayList<>();
+                            for (int i = 0; i < result.getData().size(); i++) {
+                                TreeNode treeNode1 = new TreeNode(result.getData().get(i));
+                                treeNode1.setLevel(1);
+                                treeNodes.add(treeNode1);
+                            }
+                            treeNode.setChildren(treeNodes);
+                            mView.refreshTree();
+                            imageView.animate().rotation(90).setDuration(200).start();
+                            break;
+                        }
+                        case 1: {
+                            mView.onFailureAnswer(1);
+                            break;
+                        }
+                        case 2: {
+                            mView.onFailureAnswer(2);
+                            break;
+                        }
+                        case 3: {
+                            mView.onFailureAnswer(3);
+                            break;
+                        }
                     }
-                    treeNode.setChildren(treeNodes);
-                    mView.refreshTree();
                     mView.swipeLayoutSetRefreshing(false);
-                    imageView.animate().rotation(90).setDuration(200).start();
                 }
             }.execute();
         } else {
diff --git a/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/SecondLevelNodeViewBinder.java b/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/SecondLevelNodeViewBinder.java
index cfd07c7..670fcb1 100644
--- a/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/SecondLevelNodeViewBinder.java
+++ b/app/src/main/java/ru/volgorobot/vrcatalog/nodeViewBinders/SecondLevelNodeViewBinder.java
@@ -12,6 +12,8 @@ import me.texy.treeview.TreeNode;
 import me.texy.treeview.base.BaseNodeViewBinder;
 import ru.volgorobot.vrcatalog.MainContract;
 import ru.volgorobot.vrcatalog.R;
+import ru.volgorobot.vrcatalog.additional.NetworkErrorException;
+import ru.volgorobot.vrcatalog.additional.ResultWithErrorCode;
 import ru.volgorobot.vrcatalog.model.CoreModel;
 import ru.volgorobot.vrcatalog.model.SecondLevelModel;
 import ru.volgorobot.vrcatalog.model.ThirdLevelModel;
@@ -29,7 +31,6 @@ public class SecondLevelNodeViewBinder extends BaseNodeViewBinder {
         imageView = (ImageView) itemView.findViewById(R.id.arrow_img);
         this.coreModel = new CoreModel(context);
         this.mView = mView;
-        coreModel.errorSignal.connect(mView::onFailureAnswer);
     }
 
     @Override
@@ -47,7 +48,7 @@ public class SecondLevelNodeViewBinder extends BaseNodeViewBinder {
     public void onNodeToggled(TreeNode treeNode, boolean expand) {
         if(expand) {
             SecondLevelModel secondLevelModel = (SecondLevelModel) treeNode.getValue();
-            new AsyncTask<Void, Void, ArrayList<ThirdLevelModel>>() {
+            new AsyncTask<Void, Void, ResultWithErrorCode<ArrayList<ThirdLevelModel>>>() {
                 @Override
                 protected void onPreExecute() {
                     super.onPreExecute();
@@ -55,24 +56,48 @@ public class SecondLevelNodeViewBinder extends BaseNodeViewBinder {
                 }
 
                 @Override
-                protected ArrayList<ThirdLevelModel> doInBackground(Void... voids) {
-                    ArrayList<ThirdLevelModel> thirdLevelModels = coreModel.getThirdLevelByParentID(secondLevelModel.getID());
-                    return thirdLevelModels;
+                protected ResultWithErrorCode<ArrayList<ThirdLevelModel>> doInBackground(Void... voids) {
+                    ArrayList<ThirdLevelModel> thirdLevelModels = null;
+                    try {
+                        thirdLevelModels = coreModel.getThirdLevelByParentID(secondLevelModel.getID());
+                    } catch (NetworkErrorException networkErrorException) {
+                        return new ResultWithErrorCode<>(thirdLevelModels, networkErrorException.getErrorCode());
+                    } catch (NullPointerException nullPointerException) {
+                        return new ResultWithErrorCode<>(thirdLevelModels, 3);
+                    }
+                    return new ResultWithErrorCode<>(thirdLevelModels, 0);
                 }
 
                 @Override
-                protected void onPostExecute(ArrayList<ThirdLevelModel> thirdLevelModels) {
-                    super.onPostExecute(thirdLevelModels);
-                    ArrayList<TreeNode> treeNodes = new ArrayList<>();
-                    for (int i = 0; i < thirdLevelModels.size(); i++) {
-                        TreeNode treeNode1 = new TreeNode(thirdLevelModels.get(i));
-                        treeNode1.setLevel(2);
-                        treeNodes.add(treeNode1);
+                protected void onPostExecute(ResultWithErrorCode<ArrayList<ThirdLevelModel>> result) {
+                    super.onPostExecute(result);
+                    switch (result.getErrorCode()) {
+                        case 0: {
+                            ArrayList<TreeNode> treeNodes = new ArrayList<>();
+                            for (int i = 0; i < result.getData().size(); i++) {
+                                TreeNode treeNode1 = new TreeNode(result.getData().get(i));
+                                treeNode1.setLevel(2);
+                                treeNodes.add(treeNode1);
+                            }
+                            treeNode.setChildren(treeNodes);
+                            mView.refreshTree();
+                            imageView.animate().rotation(90).setDuration(200).start();
+                            break;
+                        }
+                        case 1: {
+                            mView.onFailureAnswer(1);
+                            break;
+                        }
+                        case 2: {
+                            mView.onFailureAnswer(2);
+                            break;
+                        }
+                        case 3: {
+                            mView.onFailureAnswer(3);
+                            break;
+                        }
                     }
-                    treeNode.setChildren(treeNodes);
-                    mView.refreshTree();
                     mView.swipeLayoutSetRefreshing(false);
-                    imageView.animate().rotation(90).setDuration(200).start();
                 }
             }.execute();
         } else {
diff --git a/build.gradle b/build.gradle
index 8d3ef8e..03de3f4 100644
--- a/build.gradle
+++ b/build.gradle
@@ -5,6 +5,7 @@ buildscript {
     repositories {
         google()
         jcenter()
+        mavenCentral()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:3.2.1'
@@ -19,6 +20,7 @@ allprojects {
     repositories {
         google()
         jcenter()
+        mavenCentral()
     }
 }