在 null 上调用了 getter 'classification'

Posted 2023-02-23

【中文标题】在 null 上调用了 getter \'classification\'

【英文标题】：getter 'classification' was called on null在 null 上调用了 getter 'classification'

【发布时间】：2020-01-10 18:40:45

【问题描述】：

我大约两周大的时候开始颤抖，我不明白这个错误的真正含义。我将在下面添加堆栈：

List<DropdownMenuItem> showDropdownMenus() 
return CLASSIFICATION.map((item)
  return DropdownMenuItem(
    value: item,
    child: Text(item),
  );
).toList();

错误堆栈：

I/flutter (4403)：在构建 RegistrationPage(dirty, state: 我/颤振（4403）：RegistrationPageState#21643）： I/flutter (4403)：在 null 上调用了 getter 'classification'。 I/颤振（4403）：接收器：空 I/flutter（4403）：尝试调用：分类 我/颤动（4403）： I/flutter (4403)：当异常被抛出时，这是堆栈： I/flutter (4403): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5) I/flutter (4403): #1 RegistrationPageState.build (package:lesson2/view/registrationpage.dart:99:44) I/flutter (4403): #2 StatefulElement.build (package:flutter/src/widgets/framework.dart:4012:27) I/flutter (4403): #3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3924:15) I/flutter (4403): #4 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #5 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #6 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4053:11) I/flutter (4403): #7 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #8 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #9 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #10 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #11 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #12 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #13 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #14 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #15 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #16 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #17 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #18 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #19 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #20 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #21 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #22 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #23 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #24 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #25 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #26 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #27 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #28 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #29 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #30 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403):#31 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #32 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #33 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #34 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4053:11) I/flutter (4403): #35 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #36 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #37 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403):#38 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #39 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #40 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #41 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #42 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #43 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #44 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #45 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #46 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #47 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4053:11) I/flutter (4403): #48 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #49 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #50 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #51 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #52 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #53 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #54 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #55 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #56 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #57 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #58 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #59 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #60 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #61 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #62 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #63 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #64 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #65 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #66 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4053:11) I/flutter (4403): #67 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #68 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #69 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #70 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #71 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #72 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #73 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #74 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #75 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #76 SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5092:14) I/flutter (4403): #77 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #78 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #79 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #80 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #81 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #82 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #83 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #84 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #85 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #86 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #87 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #88 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4053:11) I/flutter (4403): #89 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #90 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #91 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #92 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #93 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #94 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3907:5) I/flutter (4403): #95 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4053:11) I/flutter (4403): #96 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3902:5) I/flutter (4403): #97 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3084:14) I/flutter (4403): #98 Element.updateChild (package:flutter/src/widgets/framework.dart:2887:12) I/flutter (4403): #99 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4875:32) I/flutter (4403): #100 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5208:17) I/flutter (4403): #101 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #102 _TheatreElement.update (package:flutter/src/widgets/overlay.dart:607:16) I/flutter (4403): #103 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #104 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #105 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #106 StatefulElement.update (package:flutter/src/widgets/framework.dart:4085:5) I/flutter (4403): #107 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #108 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #109 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #110 ProxyElement.update (package:flutter/src/widgets/framework.dart:4219:5) I/flutter（4403）：#111 _InheritedNotifierElement.update（包：flutter/src/widgets/inherited_notifier.dart:94:11） I/flutter (4403): #112 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #113 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5099:14) I/flutter (4403): #114 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #115 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #116 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #117 StatefulElement.update (package:flutter/src/widgets/framework.dart:4085:5) I/flutter (4403): #118 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #119 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5099:14) I/flutter (4403): #120 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #121 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5099:14) I/flutter (4403): #122 Element.updateChild (package:flutter/src/widgets/framework.dart:2876:15) I/flutter (4403): #123 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3935:16) I/flutter (4403): #124 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5) I/flutter (4403): #125 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2340:33) I/flutter (4403): #126 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:700:20) I/flutter (4403): #127 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:285:5) I/flutter (4403): #128 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1016:15) I/flutter (4403): #129 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:958:9) I/flutter (4403): #130 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:874:5) I/flutter (4403):#134 _invoke (dart:ui/hooks.dart:236:10) I/flutter (4403): #135 _drawFrame (dart:ui/hooks.dart:194:3) I/flutter（4403）：（从包 dart:async 中省略了 3 帧）

【参考方案1】：

您的 CLASSIFICATION 可能未正确初始化。

请查看下面的示例

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget 
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) 
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  


class MyHomePage extends StatefulWidget 
  MyHomePage(Key key, this.title) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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


List<Map> _jsonApi1 = [
  "id": 0, "name": "default 1"
];
List<Map> _jsonApi2 = [
  "id": 1, "name": "second 2",
  "id": 2, "name": "third 3"
];
List<Map> _myJson = new List.from(_jsonApi1)..addAll(_jsonApi2);

class _MyHomePageState extends State<MyHomePage> 
  String _mySelection;

  @override
  Widget build(BuildContext context) 
    return new Scaffold(
      body: SafeArea(
        child: Column(
          children: <Widget>[
            Container(
              height: 500.0,
              child: new Center(
                child: new DropdownButton<String>(
                  isDense: true,
                  hint: new Text("$_jsonApi1[0]["name"]"),
                  value: _mySelection,
                  onChanged: (String newValue) 
                    setState(() 
                      _mySelection = newValue;
                    );

                    print(_mySelection);
                  ,
                  items: _myJson.map((Map map) 
                    return new DropdownMenuItem<String>(
                      value: map["id"].toString(),
                        child: new Text(
                          map["name"],
                        ),
                    );
                  ).toList(),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  

【讨论】：

你是对的。我没有正确初始化它，但 VS 代码也没有抛出错误，所以我不知道我的代码有什么问题。我想到了。谢谢！