title: The expression doesn't evaluate to a function, so it can't be invoked when using auth.currentUser
body: <p>I'm new to flutter... I'm trying to get the state of the currently loggedIn user and notify the listeners using firebase but I'm getting this error <code>The expression doesn't evaluate to a function, so it can't be invoked.</code> at the <code>auth.currentUser</code> but I don't understand why.</p>
  <p>Below is my code.</p>
  <pre><code>_initialize() async{
      SharedPreferences prefs = await SharedPreferences.getInstance();
      bool loggedIn = prefs.getBool(LOGGED_IN) ?? false;
      if(!loggedIn){
        _status = Status.Unauthenticated;
      }else{
        await auth.currentUser().then((currentUser) async{
          _user = currentUser;
          _status = Status.Authenticated;
          _userModel = await _userServices.getUserById(currentUser.uid);
        });
  
      }
      notifyListeners();
    }
  }
  </code></pre>