如何解决此AuthStateListener问题

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;

...

public class MainActivity extends AppCompatActivity {

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;

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

        mAuth = FirebaseAuth.getInstance();

        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    // 用户已登录
                } else {
                    // 用户未登录
                }
            }
        };

        mAuth.addAuthStateListener(mAuthListener);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mAuth.removeAuthStateListener(mAuthListener);
    }
}