How to delete current user document in firestore collection?

I am adding a button in my flutter app which allows users to delete their account. I also want to delete the user document present in firestore simultaneously. I am trying to achieve this using below code but facing an error.

In below code, I am getting the user uid which is same as the user's doc name :

CollectionReference users = FirebaseFirestore.instance.collection('users');

final FirebaseAuth auth = FirebaseAuth.instance;
final User? user = auth.currentUser;
final uid = user?.uid;
final authService = Provider.of<AuthService>(context);

Now I am deleting the current user's document in the users collection:

ElevatedButton(
              onPressed: ()async{
               await users.doc(uid).delete();  //delete doc
                user!.delete().then((value) async { //delete user
                await authService.signOut();
                Navigator.pushAndRemoveUntil(
                    context,
                    MaterialPageRoute(
                    builder: (context) => LoginScreen(),
                    ),
                   (Route<dynamic> route) => false);
                    });
              },

But this throws an error when pressed:

type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

The relevant error-causing widget was
StreamBuilder<DocumentSnapshot<Object?>>
lib\screens\quiz_screen.dart:616
When the exception was thrown, this was the stack
#0      CoinsContainer.build.<anonymous closure>
lib\screens\quiz_screen.dart:630

This error repeats everywhere I have a Streambuilder. Why does this error occur and how else do I need to delete the current user document from firestore?