Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setTitle(java.lang.CharSequence)' [duplicate]

please can anyone help me to resolve this error

Here is the MainActivity.java file :

public class MainActivity extends AppCompatActivity {
    private FirebaseAuth mAuth;
    private Toolbar mToolbar;
    private ViewPager myViewPager;
    private TabLayout myTabLayout;
    private TabsPagerAdapter mTabsPagerAdapter;

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

        myViewPager = (ViewPager)findViewById(R.id.main_tabs_pager);
        mTabsPagerAdapter = new TabsPagerAdapter(getSupportFragmentManager());
        myViewPager.setAdapter(mTabsPagerAdapter);
        myTabLayout = (TabLayout)findViewById(R.id.main_tabs);
        myTabLayout.setupWithViewPager(myViewPager);


        mToolbar = (Toolbar)findViewById(R.id.main_app_bar);
        setSupportActionBar(mToolbar);

        getSupportActionBar().setTitle("myChat");
    }

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
        if (currentUser == null){
            LogOutUser();
        }
    }

    private void LogOutUser() {
        Intent startPageIntent = new Intent(MainActivity.this, StartPageActivity.class);
        startPageIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(startPageIntent);
    }


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

    public boolean onOptionsItemSelected(MenuItem item){
        super.onOptionsItemSelected(item);
        if(item.getItemId()== R.id.mainlogout_button){
            mAuth.signOut();
            LogOutUser();

        }
        return true;


    }
}

And here is the error :

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference at yassine.com.mychat.MainActivity.onCreate(MainActivity.java:40)