The method '[]' was called on null: firebase flutter - flutter

Question

I need to retrieve User Data from Firebase and use a builder to pass on the data to UI. When I run the apps, I method is called in on Null.
I tried many ways to call firebase data but I keep receive error message on provider or on calling the data NULL.
The error is most likely coming from the method _getProfileData() below.
_getProfileData(AuthNotifier authNotifier) async {
final uid = await Provider.of(context, listen: false).authNotifier.getCurrentUID();
await Provider.of(context, listen: false)
.collection('Users')
.document(uid)
.get().then((result) {
user.isAdmin = result.data['isAdmin'];
});
}
When I made the changes below by using Provider, another error appears with Provider not working.
final uid = await Provider.of<authNotifier>(context, listen: false).getCurrentUID();
I placed the getter in the API.
final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;
// GET UID
Future<String> getCurrentUID(User user, AuthNotifier authNotifier) async {
return (await _firebaseAuth.currentUser()).uid;
}
// GET CURRENT USER
Future getCurrentUser(User user, AuthNotifier authNotifier) async {
return await _firebaseAuth.currentUser();
}
Stream<String> get onAuthStateChanged => auth.onAuthStateChanged.map(
(FirebaseUser user) => user?.uid,
);
I structured User Data as below.
class User {
List favorites = [];
String documentID;
String displayName;
String email;
String password;
bool isAdmin;
User({
this.favorites,
this.documentID,
this.displayName,
this.email,
this.password,
this.isAdmin,
});
factory User.fromFirestore(DocumentSnapshot document) {
Map data = document.data;
return User(
favorites: data['favorite'] ?? [],
documentID: document.documentID,
displayName: data['displayName'] ?? '',
email: data['email'] ?? '',
isAdmin: data['isAdmin'] ?? false,
);
}
// get admin => null;
Map<String, dynamic> toMap() {
return {
'displayName': displayName,
'email': email,
'isAdmin': isAdmin,
};
}
}