error: The argument type 'UserModel? Function(User?)' can't be assigned to the parameter type 'UserModel Function(User?)' - flutter

Question

I am getting the following error in flutter.
UserModel is a class
class UserModel {
final String uid;
UserModel({this.uid});
}
And the code where this error is coming up is
Stream<UserModel> get user {
return _auth.authStateChanges()
.map(_userFromFirebaseUser);
}
Complete code:
class AuthService {
final FirebaseAuth _auth = FirebaseAuth.instance;
UserModel? _userFromFirebaseUser(User? user) {
return user != null ? UserModel(uid: user.uid) : null;
}
Stream<UserModel> get user {
return _auth.authStateChanges()
.map(_userFromFirebaseUser);
}
Future signInAnon() async {
try {
UserCredential result = await _auth.signInAnonymously();
User user = result.user!;
return _userFromFirebaseUser(user);
} catch (e) {
print(e.toString());
return null;
}
}
Future signInWithEmailAndPassword( String email, String password) async {
try {
UserCredential result = await _auth.signInWithEmailAndPassword(email: email, password: password);
User user = result.user!;
return _userFromFirebaseUser(user);
} catch(e){
print(e.toString());
return null;
}
}
Future signUpWithEmailAndPassword( String email, String password) async {
try {
UserCredential result = await _auth.createUserWithEmailAndPassword(email: email, password: password);
User user = result.user!;
return _userFromFirebaseUser(user);
} catch(e){
print(e.toString());
return null;
}
}
Future signOut() async {
try {
return await _auth.signOut();
} catch (e){
print(e.toString());
return null;
}
}
}

Answer 1

This is happening because your _userFromFirebaseUser is defined something like this,
UserModel? _userFromFirebaseUser(User? user) {
So this means that you are saying, your _userFromFirebaseUser might return a UserModel or might return a null.
One way to fix this is to make your getter return Stream<UserModel?> instead of Stream<UserModel>.
Stream<UserModel?> get user {
return _auth.authStateChanges()
.map(_userFromFirebaseUser);
}
Now your getter might return a UserModel or it might return a null.

Answer 2

I had the same problem and I'm glad I found the solution :
UserModel _userFromFirebase (User? user){
return UserModel(uid: user!.uid);
}
Stream<UserModel> get user{
return _auth.authStateChanges().map(_userFromFirebase);
}
and if _userFromFirebase appears error use:
_userFromFirebase(user.user) ;