Flutter 出现错误；运算符[]不是为对象类型定义的功能()'

我正在使用StreamBuilder将流转换为Widgets，但我在操作符中突出显示了我的字符串，并显示错误:

The operator '[]' isn't defined for the type 'Object? Function()个

代码是这样的:

void messageStream() async {
  await for (var snapshot in _firestore.collection('messages').snapshots()) {
    for (var message in snapshot.docs) {
      print(message.data());
    }
  }
}


    body: SafeArea(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          StreamBuilder<QuerySnapshot>(
            stream: _firestore.collection('messages').snapshots(),
            builder: (context, snapshot) { //This is where the second error appeared
              if (snapshot.hasData) {
                final messages = snapshot.data?.docs;
                List<Text> messageWidgets = [];
                for (var message in messages!) {
                  final messageText = message.data['text']; //These are where the errors can be found
                  final messageSender = message.data['sender'];//These are where the errors can be found.

                  final messageWidget = Text('$messageText from $messageSender');
                  messageWidgets.add(messageWidget);
                }
                return Column(
                  children: messageWidgets,
                );
              }
              return build(context);
            },
          ),