空检查运算符用于空值

Question

我需要获取数据库中级别小于或等于用户当前级别的所有消息的列表。为此，我尝试使用嵌套的streambuilder。第一个查询用户信息，然后使用用户级别进行第二个查询。我的问题是出现错误。

Null check operator used on null value

我不明白此错误。我检查了Firestore以确保集合中有数据，并且还确保制定了适当的规则。我已经尝试了此代码的几种变体，这是迄今为止唯一的一种，至少不会给我模拟器带来错误。我的模拟器显示应用栏，主体为空白。

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import '../screens/welcome_screen.dart';

class MessagesScreen extends StatefulWidget {
  static const String id = 'messages_screen';

  @override
  _MessagesScreenState createState() => _MessagesScreenState();
}

class _MessagesScreenState extends State<MessagesScreen> {
  final _firestore = FirebaseFirestore.instance;
  final _auth = FirebaseAuth.instance;
  User loggedInUser;
  int _userCurrentLevel;

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

  void getCurrentUser() {
    try {
      final user = _auth.currentUser;
      if (user != null) {
        loggedInUser = user;
      }
    } catch (e) {
      print(e);
    }
  }

  getMessages(AsyncSnapshot<QuerySnapshot> snapshot2) {
    return snapshot2.data.docs
        .map((doc) => new ListTile(title: Text(doc['from']), subtitle: Text(doc['text'])))
        .toList();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text('Darker Slate'),
        actions: <Widget>[
          IconButton(
            icon: const Icon(Icons.chat),
            tooltip: 'Messages',
            onPressed: () {},
          ),
          IconButton(
            icon: const Icon(Icons.exit_to_app),
            tooltip: 'Log Out',
            onPressed: () {
              _auth.signOut();
              Navigator.pushNamed(context, WelcomeScreen.id);
            },
          ),
        ],
      ),
      body: StreamBuilder<DocumentSnapshot>(
          stream: _firestore.collection('users')
              .doc(_auth.currentUser.uid)
              .snapshots(),
          builder: (context, snapshot) {
            if (!snapshot.hasData) {
              return Center(
                child: CircularProgressIndicator(
                  backgroundColor: Colors.red[900],
                ),
              );
            }
            _userCurrentLevel = snapshot.data['userlevel'];
            return StreamBuilder<QuerySnapshot>(
                stream: _firestore.collection('messages')
                    .where('level', isLessThanOrEqualTo: _userCurrentLevel).snapshots(),
                builder: (context, snapshot2) {
                  if (!snapshot2.hasData) {
                    return Center(
                      child: CircularProgressIndicator(
                        backgroundColor: Colors.red[900],
                      ),
                    );
                  }
                  return Column(
                    children: [
                      ListView.builder(
                          scrollDirection: Axis.vertical,
                          shrinkWrap: true,
                          itemCount: snapshot2.data.docs.length,
                          itemBuilder: (_, index) {
                            return new ListView(children: getMessages(snapshot2));
                      }),
                    ],
                  );
                }
            );
          }),
    );
  }
}