[Flutter] Error: Property 'uid' cannot be accessed on 'User?' because it is potentially null

Hi new to flutter and firebase what i am tried to make is simple signup new user then add it's information to cloud firestore with custom id and adding subcollections names "notifications" here sample of the code ..

final _auth = FirebaseAuth.instance;
final _ref = FirebaseFirestore.instance;
final userCollections = FirebaseFirestore.instance.collection("users");
Future<void> SignupToColud(
{@required String name = "",
@required String email = "",
@required String pass = ""}) async {
try {
await _auth
.createUserWithEmailAndPassword(email: email, password: pass)
.then((value) {
String userId = value.user.uid;
if (userId != null) {
userCollections.doc(userId).set({'name': name, 'email': email});
userCollections
.doc(userId)
.collection("notifications")
.add({'txt': 'welcome you registered as new user succefully'});
} else {
throw Error();
}
});
// first sign up to cloud if success

//then add to users collections
} catch (e) {
print(e.toString());
}
}


i got this error !!

lib/Sign_up.dart:24:36: Error: Property 'uid' cannot be accessed on 'User?' because it is potentially null.
- 'User' is from 'package:firebase_auth/firebase_auth.dart' ('/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_auth-1.2.0/lib/firebase_auth.dart').
Try accessing using ?. instead.
String userId = value.user.uid;
^^^


this is pubspec.yaml file information environment: sdk: ">=2.12.0 <3.0.0"

dependencies:

flutter:
sdk: flutter
cupertino_icons: ^1.0.0
firebase_core: ^1.2.0
cloud_functions: ^1.1.0
cloud_firestore: ^2.2.0
firebase_storage: ^8.1.0
firebase_auth: ^1.2.0


i really have no idea with the problem and how to solve it

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