Flutter:无法将Firestore数据存储到局部变量(返回null)

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

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  String data;

  Future<void> fetchData() async {
    DocumentSnapshot snapshot = await FirebaseFirestore.instance
        .collection('your_collection')
        .doc('your_document')
        .get();
    setState(() {
      data = snapshot.data()['your_field'];
    });
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text(data ?? 'Loading...'),
      ),
    );
  }
}