Null 检查用于 null 值的运算符

Question

I'm getting this error when i was tried to use different pages based on the user role. I don't know how should I use the? or. in the StreamBuilder or the class itself. i don't know it said that the userDoc is null.

Null check operator used on a null value

class Start extends StatelessWidget {
  const Start({super.key});

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<User?>(
      stream: FirebaseAuth.instance.authStateChanges(),
      builder: (context, snapshot) {
        if (snapshot.hasData && snapshot.data != null) {
          UserHelper.saveUser(snapshot.data!);
          return StreamBuilder<DocumentSnapshot>(
            stream: FirebaseFirestore.instance
                .collection('users')
                .doc('Tma4901vh0aXGHjbsvtp8k0fTJS2')
                .snapshots(),
            builder: (BuildContext context,
                AsyncSnapshot<DocumentSnapshot> snapshot) {
              final userDoc = snapshot.data;
              final user = userDoc!.data(); // Here is the error
              if ((user as Map<String, dynamic>)['role'] == 'admin') {
                return const Home();
              } else {
                return const UserHome();
              }
            },
          );
        } else if (snapshot.hasError) {
          return const Center(child: Text('Algo ha salido mal!'));
        } else {
          return const LoginScreen();
        }
      },
    );
  }
}

Answer 1

Wrap this code with if(snapshot.hasData)

final userDoc = snapshot.data;
final user = userDoc!.data();
  if ((user as Map<String, dynamic>)['role'] == 'admin') {
      return const Home();
  } else {
      return const UserHome();
  }

Then it would be like that:

if(snapshot.hasData){
    final userDoc = snapshot.data;
    final user = userDoc!.data();
    if ((user as Map<String, dynamic>)['role'] == 'admin') {
       return const Home();
    } else {
       return const UserHome();
    }
}else{
    return TheWidgetYouWant();
}

Answer 2

Try this:

  if(snapshot.hasData){
    final userDoc = snapshot.data;
    final user = userDoc!.data();
    if ((user as Map<String, dynamic>)['role'] == 'admin') {
       return const Home();
    } else {
       return const UserHome();
    }
  } else {
    return Container();
  }

Answer 3

if you use? this will throw an error when the value is null try using ? operator which will check if value is not null then it will try to call the method after this operator and not causing any error if used with null value

change this line

final user = userDoc!.data(); 

to this

final user = userDoc?.data();