运行时出现运行时错误（不掉毛）

Question

我遇到错误：“ package：cloud_firestore / src / firestore.dart”：失败的断言：第72行pos 12：“ path！= null”：不正确。在为我的应用程序实现提供程序后，我得到了这个。我尝试使用提供程序以及ChangeNotifierProvider，但遇到类似错误。没有起毛，所以我对哪里出问题感到困惑。

以下是我应用顶部的提供商：

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<UserModel>(
      builder: (context) => UserModel(),
      child: MaterialApp(
        title: 'Profile Demo',
        theme: new ThemeData(
          primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Profile'),
    ),
    );
  }
}

使用流生成器和使用者代码如下：

Widget build(BuildContext context) {
    final _width = MediaQuery.of(context).size.width;
    final _height = MediaQuery.of(context).size.height;
    return StreamBuilder<DocumentSnapshot>(
        stream: Firestore.instance
            .collection(Provider.of<UserModel>(context).uid)
            .document('testuser')
            .snapshots(),
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            return Consumer<UserModel>(
              builder: (context, userModel, child) => Stack(
              children: <Widget>[
                new Container(...

还附上完整的代码以供参考：

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'dart:ui' as ui;
import 'package:provider/provider.dart';
import 'package:flutter_appprofilescreenv1/user_model.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<UserModel>(
      builder: (context) => UserModel(),
      child: MaterialApp(
        title: 'Profile Demo',
        theme: new ThemeData(
          primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Profile'),
    ),
    );
  }
}

class User {
  final int name;
  final DocumentReference reference;

  User.fromMap(Map<String, dynamic> map, {this.reference})
      : name = map['name'];

  User.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);
}

class Photo {
  final int photourl;
  final DocumentReference reference;

  Photo.fromMap(Map<String, dynamic> map, {this.reference})
      : photourl = map['photourl'];

  Photo.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);
}

class Questions {
  final int totalquestions;
  final DocumentReference reference;

  Questions.fromMap(Map<String, dynamic> map, {this.reference})
      : totalquestions = map['totalquestions'];

  Questions.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    final _width = MediaQuery.of(context).size.width;
    final _height = MediaQuery.of(context).size.height;
    return StreamBuilder<DocumentSnapshot>(
        stream: Firestore.instance
            .collection(Provider.of<UserModel>(context).uid)
            .document('testuser')
            .snapshots(),
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            return Consumer<UserModel>(
              builder: (context, userModel, child) => Stack(
              children: <Widget>[
                new Container(
                  color: Colors.blue,
                ),
                new Image.network(
                  snapshot.data['photourl'].toString(),
                  fit: BoxFit.fill,
                ),
                new BackdropFilter(
                    filter: new ui.ImageFilter.blur(
                      sigmaX: 6.0,
                      sigmaY: 6.0,
                    ),
                    child: new Container(
                      decoration: BoxDecoration(
                        color: Colors.blue.withOpacity(0.9),
                        borderRadius: BorderRadius.all(Radius.circular(50.0)),
                      ),
                    )),
                new Scaffold(
                    appBar: new AppBar(
                      title: new Text(widget.title),
                      centerTitle: false,
                      elevation: 0.0,
                      backgroundColor: Colors.transparent,
                    ),
                    drawer: new Drawer(
                      child: new Container(),
                    ),
                    backgroundColor: Colors.transparent,
                    body: new Center(
                      child: new Column(
                        children: <Widget>[
                          new SizedBox(
                            height: _height / 12,
                          ),
                          new CircleAvatar(
                            radius: _width < _height ? _width / 4 : _height / 4,
                            backgroundImage: NetworkImage(snapshot.data['photourl']),
                          ),
                          new SizedBox(
                            height: _height / 25.0,
                          ),
                          new Text(
                            snapshot.data['name'],
                            style: new TextStyle(
                                fontWeight: FontWeight.bold,
                                fontSize: _width / 15,
                                color: Colors.white),
                          ),
                          new Padding(
                            padding: new EdgeInsets.only(
                                top: _height / 30,
                                left: _width / 8,
                                right: _width / 8),
                          ),
                          new Divider(
                            height: _height / 15,
                            color: Colors.white,
                          ),
                          new Row(
                            children: <Widget>[
                              rowCell(
                                  snapshot.data['totalquestions'], 'Answers'),
                              rowCell(
                                  '£ ${int.parse(snapshot.data['totalquestions']) * 2}', 'Earned'),
                            ],
                          ),
                          new Divider(
                              height: _height / 15, color: Colors.white),
                        ],
                      ),
                    ))
              ],
            ),
            );
          } else {
            return CircularProgressIndicator();
          }
        });
  }

  Widget rowCell(String count, String type) => new Expanded(
      child: new Column(
        children: <Widget>[
          new Text(
            '$count',
            style: new TextStyle(color: Colors.white),
          ),
          new Text(type,
              style: new TextStyle(
                  color: Colors.white, fontWeight: FontWeight.normal))
        ],
      ));
}