Null pointer exception on Room Database; on getDatabase() method

Question

I need to use Room database to do "Todo list" using fragments. I am very beginner on android. This is problem i have got

    java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.todolistwithroom.AppDatabase com.example.todolistwithroom.App.getDatabase()' on a null object reference
        at com.example.todolistwithroom.Todo.onViewCreated(Todo.java:32)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1471)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:802)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3273)
        at androidx.fragment.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3229)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:201)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:620)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:178)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1392)
        at android.app.Activity.performStart(Activity.java:7252)
        at android.app.ActivityThread.handleStartActivity(ActivityThread.java:2970)
        at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180)
        at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6810)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)

This is my fragment

public class Todo extends Fragment{
    AppDatabase appDatabase;
    RecyclerView recyclerView;
    RecyclerView.Adapter adapter;
    Button button;
    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.activity_main,container,false);
        return view;
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        appDatabase = App.getInstance().getDatabase();
        TodoTaskDao taskDao = appDatabase.todoTaskDao();
        recyclerView = view.findViewById(R.id.rv_todo_main);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
        adapter = new TodoAdapter(taskDao.getAll());
        recyclerView.setAdapter(adapter);
        button = view.findViewById(R.id.B_add_task_button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(getContext(),AddMainActivity.class);
                startActivity(intent);
                getActivity().finish();
            }
        });
    }
}

App class where it initialized

public class App extends Application {
    public static App instance;
    private AppDatabase database;

    @Override
    public void onCreate() {
        super.onCreate();
        instance = this;
        database = Room.databaseBuilder(this, AppDatabase.class, "database")
                .allowMainThreadQueries()
                .build();
    }

    public static App getInstance() {
        return instance;
    }

    public AppDatabase getDatabase() {
        return database;
    }
}

and self database class

package com.example.todolistwithroom;

import androidx.room.Database;
import androidx.room.RoomDatabase;
@Database(entities = {TodoTask.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
    public abstract TodoTaskDao todoTaskDao();
}

Manifest:

 <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        android:name=".App">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

If anything else neeeded to specify, it will be added. I have no clue what is going on, because my friends have the same database code.