How can I solve this "build function returned null" error?

Question

I am aware this question has been asked a few times and I have tried what was suggested, i.e. placing "return" in the right places. I have also followed the instructions in my console and tried to add an empty Container, but what happens is that my screen turns black. So here is my code:

class SettingsPage extends StatefulWidget {
  @override
  _SettingsPageState createState() => _SettingsPageState();
}

class _SettingsPageState extends State<SettingsPage> {
  @override
  Widget build(BuildContext context) {
    bool isSystemSelected;
    final stream = AllowedCallInheritedWidget.of(context).allowedCallStream;
    return StreamBuilder<String>(
      stream: stream,
      builder: (context, snapshot) {
        if (snapshot.data.toString().contains('PRIORITY_SENDERS_CONTACTS')) {
          isSystemSelected = true;
          return AlertDialog(
            backgroundColor: isSystemSelected ? Colors.blueGrey : Colors.amber,
            title: Text(snapshot.data),
          );
        } else if (snapshot.data
            .toString()
            .contains('PRIORITY_SENDERS_STARRED')) {
          isSystemSelected = true;
          return AlertDialog(
            backgroundColor: isSystemSelected ? Colors.blueGrey : Colors.amber,
            title: Text(snapshot.data),
          );
        } else if (snapshot.data.toString().contains('PRIORITY_SENDERS_ANY')) {
          isSystemSelected = true;
          return AlertDialog(
            backgroundColor: isSystemSelected ? Colors.blueGrey : Colors.amber,
            title: Text(snapshot.data),
          );
        }
      },
    );
  }
}

What I am trying to do is update android system changes in real time. Thanks for any help!

Edit: I should add that "The offending widget is: StreamBuilder"