FirebaseAuth listener giving null pointer

Issue

I am making a splash screen for my app. While loading of the splash screen I wanted to check if user is logIn or not.

If user is Login then i want to show him main activity and if not Login then i wanted to redirect him to the LoginActivitiy.

I am using firebase as my backend and i am checking the user on the Asynctask. This thing i have seen at here.

I have added the FirebaseAuth listener in onStart().

I have understood that why it is giving me error but i don't understand where to add it so that my app runs fine.

if Anyone could help me out OR give some better suggestion to do the same will be helpfull.
Thanks in adavance..

MainActivity.java

public class MainActivity extends AppCompatActivity {


    private FirebaseDatabase mDataBase;
    private DatabaseReference mUserDatabase;
    protected FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthStateListener;
    private String mCurrentUser;
    private static int SPLASH_TIME_OUT = 2000;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.welcome_screen);

        mDataBase = FirebaseDatabase.getInstance();
        mUserDatabase = mDataBase.getReference("users");
        mUserDatabase.keepSynced(true);
        startProcessing();
    }

    private void startProcessing() {
     new  userCheckOperation().execute();
    }

    protected void userIsLogIn() {
        mAuth = FirebaseAuth.getInstance();
        mAuthStateListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                if (firebaseAuth.getCurrentUser() == null) {
                    Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
                    //user won't go back
                    loginIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                    startActivity(loginIntent);
                    finish();
                }
            }
        };

    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthStateListener);
    }

    private void checkUserExists() {
        if (mAuth.getCurrentUser() != null) {
            mCurrentUser = mAuth.getCurrentUser().getUid();
            mUserDatabase.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (!dataSnapshot.hasChild(mCurrentUser)) {
                        Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
                        //user won't go back
                        setupIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                        startActivity(setupIntent);
                        finish();
                    } else {
                        if (mCurrentUser.equals("nKnlkU2fWGeLlP2QDc8CLz21Fet1")) {
                            Intent admin_mainIntent = new Intent(MainActivity.this, AdminMainActivity.class);
                            //user won't go back
                            admin_mainIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

                            startActivity(admin_mainIntent);

                            finish();
                        } else {
                            Intent user_mainIntent = new Intent(MainActivity.this, UserMainActivity.class);

                            //user won't go back
                            user_mainIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

                            startActivity(user_mainIntent);

                            finish();
                        }

                    }
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mAuthStateListener != null) {
            mAuth.removeAuthStateListener(mAuthStateListener);
        }
    }

    private class userCheckOperation extends AsyncTask<Void,Void,Void>{
        @Override
        protected Void doInBackground(Void... voids) {
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    Thread.interrupted();
                }
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void aVoid) {
            super.onPostExecute(aVoid);
            userIsLogIn();
            checkUserExists();

        }

    }

}

Log

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.lenovo.jdstudio, PID: 17986
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lenovo.jdstudio/com.example.lenovo.jdstudio.MainActivity}: java.lang.NullPointerException
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2429)
                      at android.app.ActivityThread.access$800(ActivityThread.java:151)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1342)
                      at android.os.Handler.dispatchMessage(Handler.java:110)
                      at android.os.Looper.loop(Looper.java:193)
                      at android.app.ActivityThread.main(ActivityThread.java:5333)
                      at java.lang.reflect.Method.invokeNative(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:515)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:828)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:644)
                      at dalvik.system.NativeStart.main(Native Method)
                   Caused by: java.lang.NullPointerException
                      at com.example.lenovo.jdstudio.MainActivity.onStart(MainActivity.java:94)
                      at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1174)
                      at android.app.Activity.performStart(Activity.java:5353)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2340)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2429) 
                      at android.app.ActivityThread.access$800(ActivityThread.java:151) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1342) 
                      at android.os.Handler.dispatchMessage(Handler.java:110) 
                      at android.os.Looper.loop(Looper.java:193) 
                      at android.app.ActivityThread.main(ActivityThread.java:5333) 
                      at java.lang.reflect.Method.invokeNative(Native Method) 
                      at java.lang.reflect.Method.invoke(Method.java:515) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:828) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:644) 
                      at dalvik.system.NativeStart.main(Native Method) 

Solution

Simple use this, I personally refer this method:

    //Get Firebase auth instance
    FirebaseAuth auth = FirebaseAuth.getInstance();
    if (auth.getCurrentUser() != null) {
        // User is logged in - send it to home screen \\
    } else {
        //User is not logged in - send it to login screen \\
    }

You can put this code in onCreate() method, and can send user to main activity if user is logged in, else send user to login screen

For your splash screen:

public class SplashScreenActivity extends AppCompatActivity {

    private static final int SPLASH_TIME_OUT = 2000;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_splash_screen);

        new Handler().postDelayed(new Runnable() {

            /*
             * Showing splash screen with a timer. This will be useful when you
             * want to show case your app logo / company
             */

            @Override
            public void run() {
                //Get Firebase auth instance
                FirebaseAuth auth = FirebaseAuth.getInstance();
                if (auth.getCurrentUser() != null) {
                    // User is logged in - send it to home screen \\
                } else {
                    //User is not logged in - send it to login screen \\
                }
            }
        }, SPLASH_TIME_OUT);

    } // End On-Create \\

} // End Class \\

Answered By - Rahul Singh Chandrabhan