How to implement proper authentication with graphql and flutter?

Question

I am trying to implement login authentication with graphql in flutter not sure how to proceed, plus I keep getting an error: The body might complete normally, causing null to be returned but the return type 'future or bool' is potentially non-nullable.

*** here is the graphql service

class GraphQLService {
  static GraphQLConfig graphQLConfig = GraphQLConfig();
  GraphQLClient client = graphQLConfig.clientToQuery();

  Future<bool> login(
      {String? email,
      String? phoneauth,
      String? username,
      required String password}) async {
    assert(email != null);
    QueryResult result = await client.mutate(
      MutationOptions(
        document: gql("""
mutation Mutation(\$loginInput) {login(loginInput: \$loginInput)}
"""),
        variables: {
          "loginInput": {
            'password': password,
            'email': email,
            'authPhone': phoneauth,
            'username': username,
          }
        },
      ),
    );

    if (result.hasException) {
      print(result.exception.toString());
      return true;
    }

    String token = result.data!['login']['token'];

    print(token);
  }
}

****here is the config

class GraphQLConfig {
  static HttpLink httpLink =
      HttpLink('https://fakelink.com/graphql'); ///fake link

  //creating the client
  GraphQLClient clientToQuery() =>
      GraphQLClient(link: httpLink, cache: GraphQLCache());
}

***here is the login model

class LoginModel {
  final String? email;
  final String? username;
  final String? phoneauth;
  final String password;

  LoginModel({
    this.email,
    required this.password,
    this.username,
    this.phoneauth,
  });

  Map<String, dynamic> toJson() => {
        'password': password,
        'email': email,
        'authPhone': phoneauth,
        'username': username,
      };
}