有必要阅读收集，并收集。一个人说，你需要通过Future.wait分别阅读它们。但是我的代码抛出了一个错误。请帮助我需要做什么，这样我就可以上传多个集合到一个.dart文件？

class Checking extends StatefulWidget {
  static const String id = "Checking";

  @override
  State<Checking> createState() => _CheckingState();
}

class _CheckingState extends State<Checking> {
  Future<QuerySnapshot<Map<String, dynamic>>> getthosedocuments() async {
    QuerySnapshot<Map<String, dynamic>> snapshot = await FirebaseFirestore.instance
        .collection('testfz')
        //.doc('oglavlenie')
        //.collection('subcollection')
    //.limit(1)
        .get();
    return snapshot;

  }

  Future<QuerySnapshot<Map<String, dynamic>>> getthosedocuments1() async {
    QuerySnapshot<Map<String, dynamic>> snapshot = await FirebaseFirestore.instance
        .collection('testfz')
        .doc('id')
        .collection('subcollection')
    //.limit(1)
        .get();
    return snapshot;

  }

void futures() async {
  var futures = await Future.wait([getthosedocuments(), getthosedocuments1()]);
}



  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: FutureBuilder<QuerySnapshot>(
          future: futures(),
          builder: (context, snapshotsub) {
            if (snapshotsub.connectionState == ConnectionState.waiting) {
              return CircularProgressIndicator();
            }

            final docs = snapshotsub.data!.docs;

            return ListView.builder(
              shrinkWrap: true,
              physics: ScrollPhysics(),
              itemCount: snapshotsub.data?.size,
              itemBuilder: (context, index) {
                final doc = docs.elementAt(index);

我得到的错误：
此表达式的类型为“void”，因此无法使用其值。