我想在未来的构建器中显示集合的文档 ID，但出现错误

Question

Below is the code in which i want to display documents ids from cloud firestore but i got error. Error picture is also attached below

FutureBuilder<QuerySnapshot>(
                  future: db.collection('/Exam').get(),
                  builder:
                      (BuildContext context, AsyncSnapshot asyncSnapshot) {
                    if (asyncSnapshot.hasError)
                      return Text("Error: ${asyncSnapshot.error}");
                    if (!asyncSnapshot.hasData)
                      return const CircularProgressIndicator();

                    return DropdownButton<String>(
                      isExpanded: true,
                      items: asyncSnapshot.data!.docs
                          .map(
                            (snap) => DropdownMenuItem(
                              value: snap.id,
                              child: Text(
                                snap.id.toString(),
                              ),
                            ),
                          )
                          .toList(),
                      value: _selectedexam,
                      onChanged: (String? newValue) {
                        setState(() {
                          _selectedexam = newValue!;
                          _selectedsemester = null;
                          print(_selectedexam);
                        });
                      },
                    );
                  }),

here is my error picture

Answer 1

class _MyHomePageState extends State<MyHomePage> {
final docs = ... 
String selectedId;

@overrid
initState(){
   super.initState()
   selectedId = docs.first.id;
}

 @override
 Widget build(BuildContext context) {
  final items = docs.map<DropdownMenuItem<String>>((Document value) {
        return DropdownMenuItem<String>(
          value: value.id,
          child: Text(value.id),
        );
      }).toList();

  return Container(
    child: DropdownButton<String>(
        value: selectedId,
        onChanged: (String? value) {
          setState(() {
            selectedId = value!;
          });
        },
        items: items,
      )
     ),
    );
  }
}

I edited directly in stackoverflow editor, so I may have made typos. Also consider using a FutureBuilder as suggested by @Jacob