如何检查flutter的云firestore中是否存在文档？

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

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(); // 初始化Firebase
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Firestore Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Firestore Demo'),
        ),
        body: Center(
          child: RaisedButton(
            child: Text('Check Document'),
            onPressed: () {
              checkDocument();
            },
          ),
        ),
      ),
    );
  }
}

void checkDocument() async {
  CollectionReference collection =
      FirebaseFirestore.instance.collection('your_collection'); // 替换为你的集合名称
  DocumentReference document =
      collection.doc('your_document'); // 替换为你的文档名称

  DocumentSnapshot snapshot = await document.get();
  if (snapshot.exists) {
    print('Document exists!');
  } else {
    print('Document does not exist!');
  }
}