NoSuchMethodError 被抛出构建 StreamBuilder<documentsnapshot></documentsnapshot>

Question

Actually I am following a tutorial for role based authentication. Problem is that when I first login, I am navigated to the right screen as expected and snapshot does have data! User: {role: admin, last_login: Timestamp(seconds=1610430754, nanoseconds=74000000), name: bilal, created_at: Timestamp(seconds=1610429038, nanoseconds=204000000), build_number: 1, email: bilalsaeed781833@gmail.com}

But when I create another user onto the moved screen and the same Stream builder again check for role based authentication, snapshot doesn't have data.

User:  null UserDoc: Instance of 'DocumentSnapshot'  

======== Exception caught by widgets library =======================================================
The following NoSuchMethodError was thrown building StreamBuilder<DocumentSnapshot>(dirty, state: _StreamBuilderBaseState<DocumentSnapshot, AsyncSnapshot<DocumentSnapshot>>#0f4cc):
The method '[]' was called on null.
Receiver: null
Tried calling: []("role")

I don't know why its giving that error!

Here is my code:

return StreamBuilder<User>(
        stream: FirebaseAuth.instance.authStateChanges(),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.active) {
            return StreamBuilder<DocumentSnapshot>(
              stream: FirebaseFirestore.instance
                  .collection("users")
                  .doc(snapshot.data.uid)
                  .snapshots(),
              builder: (BuildContext context,
                  AsyncSnapshot<DocumentSnapshot> snapshot) {
                if (snapshot.connectionState == ConnectionState.active) {
                  final userDoc = snapshot.data;
                  final user = userDoc.data();
                  debugPrint("User:  ${user.toString()} UserDoc: ${userDoc.toString()}  ");
                  if (user['role'] == 'admin') {
                    return admin();

                  } else if (user['role'] == 'hod') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => hod()),
                    );
                  } else if (user['role'] == 'duo') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => duo()),
                    );
                  } else if (user['role'] == 'teacher') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => teacher()),
                    );
                  } else if (user['role'] == 'cr') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => cr()),
                    );
                  } else if (user['role'] == 'student') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => student()),
                    );
                  } else {
                    return Material(
                      child: Center(
                        child: CircularProgressIndicator(),
                      ),
                    );
                  }
                }

                return Material(
                  child: Center(
                    child: CircularProgressIndicator(),
                  ),
                );
              },
            );
          }
          return Material(
            child: Center(
              child: CircularProgressIndicator(),
            ),
          );
        });

Answer 1

Wrap everything inside your StreamBuilder in an if statement.

if (snapshot.connectionState == ConnectionState.active) {
  if(snapshot.hasData){
     ...
  } else {
      return CircularProgressIndicator()
  }
}

This should prevent the builder from trying to build something when it has no data.