android - 如何解决此“构建函数返回 null”错误？

Question

我知道这个问题已经被问过几次，我已经尝试过建议的方法，即将“返回”放在正确的位置。我也按照控制台中的说明尝试添加一个空容器，但结果是我的屏幕变黑了。所以这是我的代码：

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),
          );
        }
      },
    );
  }
}

我想做的是实时更新android系统的变化。谢谢你的帮助！

编辑：我应该添加“有问题的小部件是：StreamBuilder”