我在Firestore中有一个名为Todos的集合，它有3个可能的属性（id，text和checked）。到目前为止，我已经成功地创建并保存了todo。现在我想将它们放在列表视图中，但在热启动时返回错误：

════════ Exception caught by widgets library ═══════════════════════════════════
type 'Null' is not a subtype of type 'String'
The relevant error-causing widget was
StreamBuilder<QuerySnapshot<Object?>>

显示列表视图的代码：

final Stream<QuerySnapshot> _todostream =
      FirebaseFirestore.instance.collection('Todos').snapshots();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Styles.bgColor,
      floatingActionButton: FloatingActionButton(
        onPressed: createNewTask,
        child: const Icon(Icons.add),
      ),
      body: StreamBuilder(
        stream: _todostream,
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (snapshot.hasError) {
            return const Text('Something went wrong');
          }
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const CircularProgressIndicator();
          }
          return ListView(
            children: snapshot.data!.docs.map((DocumentSnapshot document) {
              Map<String, dynamic> data =
                  document.data()! as Map<String, dynamic>;

              return ListTile(title: Text(data['text']));
            }).toList(),
          );
        },
      ),
    );
  }
}

我希望看到一个列表视图与我的收集结果，它包含2个项目。