我是Flutter的新手，我正在尝试开发一个应用程序。
我想显示Firebase数据库中的人员列表。但是，我得到了以下错误。

• • 错误：**

无法无条件调用方法"[]"，因为接收方可以为"null"。请尝试使调用具有条件（使用"？."）或向目标（"!"）添加null检查。
科达勒姆：

`import 'package:calendar/page/mainPage.dart';
import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

class Staff extends StatefulWidget {
  @override
  _StaffState createState() => _StaffState();
}

class _StaffState extends State<Staff> {
  
  final _firestore = FirebaseFirestore.instance;

  @override
  Widget build(BuildContext context) {
    // ignore: unused_local_variable
    CollectionReference staffRef = _firestore.collection('staff');

    return Scaffold(
      appBar: AppBar(
        title: Text("Personel Listesi"),
        backgroundColor: Colors.redAccent[400],
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.home),
            onPressed: () {
              Navigator.pushAndRemoveUntil(
                  context,
                  MaterialPageRoute(builder: (_) => MainPage()),
                  (route) => true);
            },
          ),
        ],
      ),
      body: Container(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Center(
            child: Column(
              children: [
                StreamBuilder<QuerySnapshot>(
                  stream: staffRef.snapshots(),
                  builder: (BuildContext context, AsyncSnapshot asyncSnapshot) {
                    if (asyncSnapshot.hasError) {
                      return Center(
                          child: Text(
                              "Bir hata oluştu, lütfen tekrar deneyiniz."));
                    } else {
                      if (asyncSnapshot.hasData) {
                        List<DocumentSnapshot> listStaff =
                            asyncSnapshot.data.docs;
                        return Flexible(
                          child: ListView.builder(
                              itemBuilder: (context, index) {
                                return Card(
                                  elevation: 20,
                                  color: Colors.greenAccent[200],
                                  child: ListTile(
                                    trailing: IconButton(
                                      icon: Icon(Icons.delete),
                                      onPressed: () async {
                                        await listStaff[index]
                                            .reference
                                            .delete();
                                      },
                                    ),
                                    title: Text(
                                      '${listStaff[index].data['nameSurname']}',
                                      style: TextStyle(fontSize: 20),
                                    ),
                                    subtitle: Column(
                                      children: [
                                        Row(
                                          mainAxisAlignment:
                                              MainAxisAlignment.start,
                                          children: [
                                            Text(
                                              '${listStaff[index].data['tip']}',
                                              style: TextStyle(fontSize: 14),
                                            ),
                                          ],
                                        ),
                                        Row(
                                          mainAxisAlignment:
                                              MainAxisAlignment.start,
                                          children: [
                                            Text(
                                              '${listStaff[index].data['mail']}',
                                              style: TextStyle(fontSize: 14),
                                            ),
                                          ],
                                        ),
                                        Row(
                                          mainAxisAlignment:
                                              MainAxisAlignment.start,
                                          children: [
                                            Text(
                                              '${listStaff[index].data['phone']}',
                                              style: TextStyle(fontSize: 14),
                                            ),
                                          ],
                                        ),
                                      ],
                                    ),
                                  ),
                                );
                              },
                              itemCount: listStaff.length),
                        );
                      } else {
                        return Center(
                          child: CircularProgressIndicator(),
                        );
                      }
                    }
                  },
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}
`