From the two same lines, one is generating an error relative to FutureOrNull - dart

Question

Thos are the two function from the same class I wrote: The first has the error:
A value of type 'Future<String?>' can't be returned by the 'onError' handler because it must be assignable to 'FutureOr'.
while the second is OK and I don't understand why.
Can you tell me please?
/// Creates or update the User collection
Future<String?> _createOrUpdateUser(User user) {
final email = user.email;
final nameFromEmail =
email!.substring(0, email.indexOf('#')).replaceAll(".", " ").trim();
return FirebaseFirestore.instance.collection('users').doc(user.uid).set({
'id': user.uid,
'screenName': '',
'displayName': user.displayName ?? nameFromEmail,
'photoUrl': user.photoURL,
'bio': '',
'darkMode': false,
'email': user.email,
}).then((_) {
debugPrint("User ${user.displayName} is created");
return null;
}).catchError((e) {
debugPrint(e.toString());
return Future<String?>.error(e); // <<<<< Error here: A value of type 'Future<String?>' can't be returned by the 'onError' handler because it must be assignable to 'FutureOr<Null>'.
});
}
Future<String?> _authUser(BuildContext context, LoginData data) {
debugPrint('Name: ${data.name}, Password: ${data.password}');
return auth
.signInWithEmailAndPassword(email: data.name, password: data.password)
.then((credential) {
try {
final user = credential.user;
if (user == null) {
return 'Password does not match';
} else {
_createOrUpdateUser(user);
// == Tell the app we just signed in
Provider.of<StateModel>(context, listen: false).signedIn = user;
return null;
}
} catch (e) {
return 'User not exists';
}
}).catchError((e) {
if (e.code == 'weak-password') {
debugPrint('The password provided is too weak.');
} else if (e.code == 'email-already-in-use') {
debugPrint('The account already exists for that email.');
} else {
debugPrint(e.toString());
}
return Future<String?>.error(e); // <<<<< No problem here
});
}