如何使用列表视图生成器flutter null安全中的firebase集合长度

var collectionRef = FirebaseFirestore.instance.collection('collectionName');
var snapshot = await collectionRef.get();
var documents = snapshot.docs;

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Firebase Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Firebase Collection Length'),
      ),
      body: Center(
        child: FutureBuilder<QuerySnapshot>(
          future: FirebaseFirestore.instance.collection('collectionName').get(),
          builder: (context, snapshot) {
            if (snapshot.connectionState == ConnectionState.waiting) {
              return CircularProgressIndicator();
            }
            if (snapshot.hasError) {
              return Text('Error: ${snapshot.error}');
            }
            var documents = snapshot.data!.docs;
            var collectionLength = documents.length;
            return Text('Collection Length: $collectionLength');
          },
        ),
      ),
    );
  }
}