Приложение внезапно перестает работать и завершает работу при переходе от активности входа в систему к основной активности

Question

Я работал над клоном WhatsApp в Android Studio. после запуска на физическом устройстве я могу видеть активность входа в систему, и после того, как я даю учетные данные и нажимаю кнопку, он переключается на основную активность и выходит через 2 секунды. Почему это так?

Вот код основной деятельности

import android.content.DialogInterface;
import android.content.Intent;
import androidx.annotation.NonNull;
import com.google.android.material.tabs.TabLayout;
import androidx.viewpager.widget.ViewPager;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import androidx.appcompat.widget.Toolbar;
import android.text.TextUtils;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.HashMap;

public class MainActivity extends AppCompatActivity {

    private Toolbar mToolbar;
    private ViewPager myViewPager;
    private TabLayout myTabLayout;
    private TabsAccessorAdapter mTabsAccessorAdapter;

    private FirebaseUser currentUser;
    private FirebaseAuth mAuth;
    private DatabaseReference RootRef;

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

        mAuth = FirebaseAuth.getInstance();
        currentUser = mAuth.getCurrentUser();
        RootRef = FirebaseDatabase.getInstance().getReference();





        mToolbar = findViewById(R.id.main_page_toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setTitle("WhatsApp");

        myViewPager = (ViewPager) findViewById(R.id.main_tabs_pager);
        mTabsAccessorAdapter = new TabsAccessorAdapter(getSupportFragmentManager());
        myViewPager.setAdapter(mTabsAccessorAdapter);

        myTabLayout = (TabLayout) findViewById(R.id.main_tabs);
        myTabLayout.setupWithViewPager(myViewPager);
    }

    @Override
    protected void onStart() {
        super.onStart();
        if (currentUser == null) {
            SendUserToLoginActivity();
        } else {
            VerifyUserExist();
        }
    }

    private void VerifyUserExist() {
        String currentUserID = mAuth.getCurrentUser().getUid();
        RootRef.child("Users").child(currentUserID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if ((dataSnapshot.child("name").exists())) {
                    Toast.makeText(MainActivity.this, "Welcome", Toast.LENGTH_SHORT).show();
                } else {
                    SendUserToSettingsActivity();
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        super.onCreateOptionsMenu(menu);
        getMenuInflater().inflate(R.menu.options_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        super.onOptionsItemSelected(item);

        if (item.getItemId() == R.id.main_logout_option) {
            mAuth.signOut();
            SendUserToLoginActivity();
        }

        if (item.getItemId() == R.id.main_create_group_option) {
            RequestNewGroup();

        }
        if (item.getItemId() == R.id.main_settings_option) {
            SendUserToSettingsActivity();

        }

        if (item.getItemId() == R.id.main_find_friends_option) {
            SendUserToFindFriendsActivity();

        }

        return true;
    } 

    private void RequestNewGroup() {
        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this, 
R.style.AlertDialog);
        builder.setTitle("Enter Group Name :");

        final EditText groupNameField = new EditText(MainActivity.this);
        groupNameField.setHint("e.g Coding Cafe");
        builder.setView(groupNameField);

        builder.setPositiveButton("Create", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                String groupName = groupNameField.getText().toString();
                if (TextUtils.isEmpty(groupName)) {
                    Toast.makeText(MainActivity.this, "Please write Group Name..", 
Toast.LENGTH_SHORT).show();
                } else {
                    CreateNewGroup(groupName);

                }

            }
        });

        builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int which) {
                dialogInterface.cancel();

            }
        });

        builder.show();
    }

    private void CreateNewGroup(final String groupName) {
        RootRef.child("Groups").child(groupName).setValue("")
                .addOnCompleteListener(new OnCompleteListener<Void>() {
                    @Override
                    public void onComplete(@NonNull Task<Void> task) {
                        if (task.isSuccessful()) {
                            Toast.makeText(MainActivity.this, groupName + " is created 
successfully.", Toast.LENGTH_SHORT).show();
                        }

                    }
                });
    }

    private void SendUserToLoginActivity() {
        Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
        loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(loginIntent);
    }

    private void SendUserToSettingsActivity() {
        Intent settingsIntent = new Intent(MainActivity.this, SettingsActivity.class);
        startActivity(settingsIntent);
    }

    private void SendUserToFindFriendsActivity() {
        Intent findFriendsIntent = new Intent(MainActivity.this, FindFriendsActivity.class);
        startActivity(findFriendsIntent);
    } 

    private void SendUserToChatActivity() {
        Intent findFriendsIntent = new Intent(MainActivity.this, ChatActivity.class);
        startActivity(findFriendsIntent);
    }
}

манифест

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.chatbuddy">

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <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">
        <activity android:name=".LoginActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".MainActivity" />
        <activity android:name=".RegisterActivity" />
        <activity
            android:name=".SettingsActivity"
            android:parentActivityName=".MainActivity" />
        <activity android:name=".GroupChatActivity" />
        <activity android:name=".PhoneLoginActivity" />
        <activity
            android:name="com.theartofdev.edmodo.cropper.CropImageActivity"
            android:theme="@style/Base.Theme.AppCompat" />
        <activity
            android:name=".FindFriendsActivity"
            android:parentActivityName=".MainActivity" />
        <activity android:name=".ProfileActivity" />

        <activity android:name=".ChatActivity"
            android:parentActivityName=".MainActivity" />
   </application>

</manifest>