private void loginViaEmailPassword(String email, String pass) {
        showProgressBar();
        try {
             // check if user is registered. then try login
            FirebaseAuth.getInstance().fetchSignInMethodsForEmail(email).addOnCompleteListener(task -> {
                SignInMethodQueryResult result = task.getResult();
                if (task.isSuccessful() && result != null && result.getSignInMethods() != null && result.getSignInMethods().size() > 0) {
                    // user is registered, now try login
                    tryLogIn(email, pass, new OnFireBaseLogin() {
                        @Override
                        public void onSuccess(FirebaseUser user) {
                            // check if email is verified, if not send verification email.
                            if (user.isEmailVerified()) {
                                // user is verified, redirect to main screen
                                startMainActivity();
                            } else {
                                sendVerificationEmail(user, task1 -> {
                                    hideProgressBar();
                                    FirebaseAuth.getInstance().signOut();
                                    if (task1.isSuccessful()) {
                                        Utilities.getInstance().showDialog(ActivityLogin.this, getString(R.string.sent_verification_email), getString(R.string.msg_sent_verification_email), (dialog, which) -> {
                                            dialog.dismiss();
                                        });
                                    } else {
                                        errorMessage(getString(R.string.msg_error_sending_email));
                                    }
                                });
                            }
                        }

                        @Override
                        public void onError(int error, @Nullable Throwable exception) {
                            hideProgressBar();
                            assert exception != null;
                            {
                                App.getInstance().logException(new Exception(exception), getClass());
                                errorMessage(exception.getMessage());
                            }
                        }
                    });
                } else {
                    hideProgressBar();
                    errorMessage(getString(R.string.msg_email_not_registered));
                }
            });
        } catch (Exception e) {
            hideProgressBar();
            App.getInstance().logException(e, getClass(), true);
            Utilities.getInstance().showDialog(this, getString(R.string.some_error_occurred), getString(R.string.msg_fail_login_play_service)).show();
        }
    }

    public void tryLogIn(String email, String pass) {
        FirebaseAuth.getInstance().signInWithEmailAndPassword(email, pass).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                FirebaseUser currentUser = FirebaseAuth.getInstance().getCurrentUser();
                if (task.isSuccessful() && currentUser != null) {
                    Log.w(TAG, "signInWithCustomToken:success", task.getException());
                    successResponse(currentUser);
                } else {
                    Log.w(TAG, "signInWithCustomToken:failure", task.getException());
                    errorResponse(0, task.getException());
                }
            }
        });
    }