How To Prevent Users From Logging In An Android App, If They Don't Click On The Verification Link Sent By Firebase Authentication First?

I am making an android chat application. It opens up with a StartActivity page that leads to either of the Login or Register pages. I am using Firebase authentication system, i.e.

Solution 1:

The NullPointerException is caused by this code:

public class StartActivity extends AppCompatActivity {

    MaterialButton goToLogin, goToRegister;
    ImageView banner;

    FirebaseUser firebaseUser;
    FirebaseAuth auth;

    @Override
    protected void onStart() {
        super.onStart();

        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();

        //redirect if user is not null
        if (Objects.requireNonNull(auth.getCurrentUser()).isEmailVerified()) {

You never initialize auth before the call to auth.getCurrentUser(), which is why you get a NullPointerException. The simple fix is to initialize auth:

protected void onStart() {
    super.onStart();

    auth = FirebaseAuth.getInstance();

    firebaseUser = auth.getCurrentUser();

    //redirect if user is not null
    if (Objects.requireNonNull(auth.getCurrentUser()).isEmailVerified()) {
        ...

But I also highly recommend reading What is a NullPointerException, and how do I fix it?, as this type of error is quite common and an easily be fixed by following the approach outlined there.