Error Assigning 'GoogleSignInAccount?' to 'GoogleSignInAccount' in Flutter Authentication

Question

I'm integrating Google sign-in with Firebase authentication in my Flutter app. When trying to assign the result of GoogleSignIn().signIn() to a GoogleSignInAccount variable, I encounter a type mismatch error. Here's the relevant part of my code:

// ignore: import_of_legacy_library_into_null_safe
import 'package:google_sign_in/google_sign_in.dart';
import 'package:firebase_auth/firebase_auth.dart';

Future<Map<String, dynamic>> signInWithGoogle() async {
  final GoogleSignInAccount googleUser = await GoogleSignIn().signIn();
  final GoogleSignInAuthentication googleAuth = await googleUser.authentication;
  final credential = GoogleAuthProvider.credential(
    accessToken: googleAuth.accessToken,
    idToken: googleAuth.idToken,
  );

  return {
    "email": googleUser.email,
    "photoUrl": googleUser.photoUrl,
    "credentials": await FirebaseAuth.instance.signInWithCredential(credential),
  };
}

Future<bool> signInWithEmail(String email, String password) async {
  try {
    FirebaseAuth.instance.
    signInWithEmailAndPassword(email: email, password: password);
    return true;
  } on FirebaseAuthException catch(e){
    print(e.code);
    return false;
  }
}



Future<bool> signUpWithEmail(String email, String password) async {
  try {
    FirebaseAuth.instance.
    createUserWithEmailAndPassword(email: email, password: password);
    return true;
  } on FirebaseAuthException catch(e){
    print(e.code);
    return false;
  }
}

Flutter's suggestion is:

A value of type 'GoogleSignInAccount?' can't be assigned to a variable of type 'GoogleSignInAccount'. Try changing the type of the variable, or casting the right-hand type to 'GoogleSignInAccount'.

I've attempted to cast the variable to the non-nullable type but still face issues. How can I correctly assign a value of type 'GoogleSignInAccount?' to a variable of type 'GoogleSignInAccount' in this context?

Answer 1

As the error said, GoogleSignIn().signIn() returns you GoogleSignInAccount? that means it could be null and you can't pass it to variable type GoogleSignInAccount. You need to change googleUser's type to GoogleSignInAccount?.

final GoogleSignInAccount? googleUser = await GoogleSignIn().signIn();

after that you need to change your return map in signInWithGoogle to this:

final GoogleSignInAuthentication? googleAuth = await googleUser?.authentication;

...
return {
    "email": googleUser?.email ?? '',
    "photoUrl": googleUser?.photoUrl ?? '',
    "credentials": await FirebaseAuth.instance.signInWithCredential(credential),
  };

what I did is that use ? on googleUser, and set the default value (' ') incase that googleUser was null. you need to repeat this for credential too and provide it default value that good for it incase it get null too.