Getting error "the method [""] was called on null.

[NayeemCseMu]

I have to write an authentication code in separate class Auth and extend it with AuthBase class which is an abstract class.

The below code is from my authentication class.

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/cupertino.dart';

class UserModel {
  final String userId;
  UserModel({@required this.userId});
}

abstract class AuthBase {
  Stream<UserModel> get onAuthStateChanged;
  Future<UserModel> currentUser();
  Future<UserModel> signAnonymously();
  Future<void> signOut();
}

class Auth implements AuthBase {
  final _firebase = FirebaseAuth.instance;

  UserModel _userFromFirebase(User user) {
    if (user == null) {
      return null;
    }
    return UserModel(userId: user.uid);
  }

  @override
  Future<UserModel> currentUser() async {
    final user = _firebase.currentUser;
    return _userFromFirebase(user);
  }

  Stream<UserModel> get onAuthStateChanged {
    return _firebase.authStateChanges().map((_userFromFirebase));
  }

  @override
  Future<UserModel> signAnonymously() async {
    final auth = await _firebase.signInAnonymously();
    print(auth.user.uid);
    return _userFromFirebase(auth.user);
  }

  @override
  Future<void> signOut() async {
    await _firebase.signOut();
  }
}

this is my initial page from where I would call the method in Auth class,

import 'package:flutter/material.dart';
import 'package:time_tracker_app/services/authentication.dart';
import 'package:time_tracker_app/view/auth_screen/auth_screen.dart';
import 'package:time_tracker_app/view/home/home_page.dart';

class LandingPage extends StatefulWidget {
  static const String landing_page = '/landing_page';

  final AuthBase authBase;
  LandingPage({this.authBase});
  @override
  _LandingPageState createState() => _LandingPageState();
}

class _LandingPageState extends State<LandingPage> {
  UserModel _user;
  @override
  void initState() {
    super.initState();
    _checkCurrentUser();
    widget.authBase.onAuthStateChanged.listen((user) {
      print('user: ${user?.userId}');
    });
  }

  void _updateUser(UserModel user) {
    setState(() {
      _user = user;
    });
  }

  Future<void> _checkCurrentUser() async {
    UserModel user = await widget.authBase.currentUser();
    _updateUser(user);
  }

  @override
  Widget build(BuildContext context) {
    if (_user == null) {
      return AuthScreen(
        onSignIn: _updateUser,
        authBase: widget.authBase,
      );
    } else {
      return HomeScreen(
        authBase: widget.authBase,
        onSignOut: () => _updateUser(null),
      );
    }
  }
}

The problem I get ;

( 5927): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'currentUser' was called on null.
E/flutter ( 5927): Receiver: null
E/flutter ( 5927): Tried calling: currentUser()

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