在flutter中使用orderBy()时，firebase firestore中没有值

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

void main() {
  runApp(MyApp());
  Firestore.instance.settings(timestampsInSnapshotsEnabled: true);
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Firestore OrderBy Example'),
        ),
        body: FirestoreList(),
      ),
    );
  }
}

class FirestoreList extends StatefulWidget {
  @override
  _FirestoreListState createState() => _FirestoreListState();
}

class _FirestoreListState extends State<FirestoreList> {
  List<DocumentSnapshot> _data = [];

  @override
  void initState() {
    super.initState();
    _loadData();
  }

  Future<void> _loadData() async {
    QuerySnapshot querySnapshot = await Firestore.instance.collection('yourCollection').orderBy('yourField', Query.Direction.asc).get();
    setState(() {
      _data = querySnapshot.documents;
    });
  }

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: _data.length,
      itemBuilder: (context, index) {
        return ListTile(
          title: Text(_data[index]['yourField']),
        );
      },
    );
  }
}