FirebaseFirestore не позволяет мне запустить команду для инициализации и ее экземпляр

Question

Я хочу открыть вход в Google через Firebase, поймать данные пользователя, а затем добавить их в базу данных Firestore, но всякий раз, когда я набираю строку

FirebaseFirestore db = FirebaseFirestore.getInstance();

Я получаю эту ошибку:

 java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=2, result=-1, data=Intent { (has extras) }} to activity {com.gamecodeschool.notify/com.gamecodeschool.notify.WelcomePage}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.firebase.auth.internal.zzab.zzf(int)' on a null object reference

Вот остаток кода

SignInButton button;
FirebaseAuth mAuth;
private final static Integer RC_SIGN_IN = 2;
GoogleSignInClient mGoogleSignInClient;
private final static String TAG = "Eorror";


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

    button = (SignInButton) findViewById(R.id.googleBtn);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            switch (v.getId()) {
                case R.id.googleBtn:
                    signIn();
                    break;
            }

        }
    });





    GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.default_web_client_id))
            .requestEmail()
            .build();

    mGoogleSignInClient = GoogleSignIn.getClient(this, gso);

}

@Override
protected void onStart() {
    super.onStart();
    GoogleSignInAccount account = GoogleSignIn.getLastSignedInAccount(this);
    //Make UI update by passing the "account" variable which will be non null if signed in.
}

private void signIn() {
    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_IN);
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    // Result returned from launching the Intent from GoogleSignInClient.getSignInIntent(...);
    if (requestCode == RC_SIGN_IN) {
        // The Task returned from this call is always completed, no need to attach
        // a listener.
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        handleSignInResult(task);
    }
}
private void handleSignInResult(Task<GoogleSignInAccount> completedTask) {
    try {
        GoogleSignInAccount account = completedTask.getResult(ApiException.class);

        // Signed in successfully, show authenticated UI.
        //Save to database
        FirebaseFirestore db = FirebaseFirestore.getInstance();
        Map<String, Object> user = new HashMap<>();
        user.put("first", "Ada");
        user.put("last", "Lovelace");
        user.put("born", 1815);

// Добавить новый документ с сгенерированным идентификатором

        db.collection("users")
                .add(user)
                .addOnSuccessListener(new OnSuccessListener<DocumentReference>() {
                    @Override
                    public void onSuccess(DocumentReference documentReference) {
                        Log.d(TAG, "DocumentSnapshot added with ID: " + documentReference.getId());
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Log.w(TAG, "Error adding document", e);
                    }
                });
        //End of save to database
       startActivity(new Intent(WelcomePage.this, HomePage.class));
        Toast.makeText(this, "Successfully Signed In",
                Toast.LENGTH_LONG).show();
        getEmailAddress();

    } catch (ApiException e) {
        // The ApiException status code indicates the detailed failure reason.
        // Please refer to the GoogleSignInStatusCodes class reference for more information.
        Log.w(TAG, "signInResult:failed code=" + e.getStatusCode());
       // updateUI(null);
    }
}

public void getEmailAddress()
{
    GoogleSignInAccount acct = GoogleSignIn.getLastSignedInAccount(WelcomePage.this);
    if (acct != null) {
        String personName = acct.getDisplayName();
        String personEmail = acct.getEmail();
        String personId = acct.getId();

    }


}

}

Пожалуйста, объясните, почему я получаю эту ошибку? Заранее спасибо!