以下是“我的应用程序”屏幕：

import 'package:api_call_sample/Repository/JokeRepository.dart';
import 'package:api_call_sample/bloc/jokeevent.dart';
import 'package:api_call_sample/bloc/jokestate.dart';
import 'package:flutter/material.dart';
import "package:api_call_sample/bloc/jokebloc.dart";
import 'package:flutter_bloc/flutter_bloc.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: BlocProvider(
          create: (context) => JokeBloc(
                RepositoryProvider.of<JokeRepository>(context),
              )..add(LoadJokeEvent()),
          child: const HomeScreen()),
    );
  }
}

这是主屏幕屏幕：在Bloc Builder屏幕中出现错误

class HomeScreen extends StatefulWidget {
          const HomeScreen({Key? key}) : super(key: key);
        
          @override
          State<HomeScreen> createState() => _HomeScreenState();
        }
        
    
        class _HomeScreenState extends State<HomeScreen> {
          @override
          Widget build(BuildContext context) {
            return Scaffold(
              appBar: AppBar(
                title: const Text("Joke Api"),
              ),
              **body: Column( children: [**
                **BlocBuilder<JokeBloc, JokeState>(builder: (context, state) {**
                  print(state);
                  if (state is JokeLoadingState) {
                    print("Loading state");
                    return const CircularProgressIndicator();
                  }
                  if (state is JokeLoadedState) {
                    print(state.jokeData.toString());
                    return ExpansionTile(
                      title: Text(state.jokeData.setup),
                      children: [
                        Container(
                            margin: const EdgeInsets.all(5.0),
                            child: Text(state.jokeData.delivery))
                      ],
                    );
                  }
                  if (state is JokeErrorState) {
                    print("Error state");
                    return Text(state.error.toString());
                  }
                  return Container();
                }),
                ElevatedButton(
                    onPressed: () {
                      BlocProvider.of<JokeBloc>(context).add(LoadJokeEvent());
                    },
                    child: const Text('Load More'))
              ]),
            );
          }
        }

下面是JokeBloc：

import 'package:bloc/bloc.dart';
    import 'jokestate.dart';
    import 'jokeevent.dart';
    import 'package:api_call_sample/Repository/JokeRepository.dart';
    
    
    
    class JokeBloc extends Bloc<JokeEvent,JokeState>{
      
     final JokeRepository _jokeRepository;
    
      JokeBloc(this._jokeRepository) : super(JokeLoadingState())
       {
        on<LoadJokeEvent>((event, emit) async {
          emit(JokeLoadingState());
          try {
            final joke = await _jokeRepository.getJoke();
            emit(JokeLoadedState(joke));
          } catch (e) {
            emit(JokeErrorState(e.toString()));
          }
        });
      }
    }

错误消息：

The following _CastError was thrown building _BodyBuilder:
type 'Null' is not a subtype of type 'JokeBloc' in type cast.
    
The relevant error-causing widget was:Scaffold Scaffold:file:///Applications/flutter/api_call_sample/lib/main.dart:37:12
    
The relevant error-causing widget was:
  Scaffold Scaffold:file:///Applications/flutter/api_call_sample/lib/main.dart:37:12

When the exception was thrown, this was the stack:
#0      _CreateInheritedProviderState.value (package:provider/src/inherited_provider.dart:786:72)
#1      _CreateInheritedProviderState.debugFillProperties (package:provider/src/inherited_provider.dart:806:44)
#2      _InheritedProviderScopeElement.debugFillProperties (package:provider/src/inherited_provider.dart:631:20)
#3      DiagnosticableNode.builder.<anonymous closure> (package:flutter/src/foundation/diagnostics.dart:2992:17)
#4      DiagnosticableNode.builder (package:flutter/src/foundation/diagnostics.dart:2995:8)
#5      DiagnosticableNode.getProperties (package:flutter/src/foundation/diagnostics.dart:3009:105)
#6      TextTreeRenderer._debugRender (package:flutter/src/foundation/diagnostics.dart:1244:63)
#7      TextTreeRenderer.render (package:flutter/src/foundation/diagnostics.dart:1121:14)
#8      DiagnosticsNode.toStringDeep.<anonymous closure> (package:flutter/src/foundation/diagnostics.dart:1776:9)
#9      DiagnosticsNode.toStringDeep (package:flutter/src/foundation/diagnostics.dart:1783:6)
#10     DiagnosticsNode.toString.<anonymous closure> (package:flutter/src/foundation/diagnostics.dart:1694:18)
#11     DiagnosticsNode.toString (package:flutter/src/foundation/diagnostics.dart:1706:6)
#12     Diagnosticable.toString.<anonymous closure> (package:flutter/src/foundation/diagnostics.dart:3168:78)
#13     Diagnosticable.toString (package:flutter/src/foundation/diagnostics.dart:3170:6)
#14     _StringBase._interpolate (dart:core-patch/string_patch.dart:853:19)
#15     RepositoryProvider.of (package:flutter_bloc/src/repository_provider.dart:78:12)
#16     MyApp.build.<anonymous closure> (package:api_call_sample/main.dart:20:36)
#17     _CreateInheritedProviderState.value (package:provider/src/inherited_provider.dart:736:36)
#18     _InheritedProviderScopeElement.value (package:provider/src/inherited_provider.dart:590:33)
#19     Provider.of (package:provider/src/provider.dart:303:37)
#20     ReadContext.read (package:provider/src/provider.dart:649:21)
#21     _BlocBuilderBaseState.initState (package:flutter_bloc/src/bloc_builder.dart:130:36)
#22     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4942:57)
#23     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4781:5)
#24     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3817:16)
#25     MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6350:36)
#26     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6362:32)
...     Normal element mounting (19 frames)
#45     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3817:16)
#46     MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6350:36)
#47     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6362:32)
...     Normal element mounting (99 frames)
#146    _InheritedProviderScopeElement.mount (package:provider/src/inherited_provider.dart:411:11)
...     Normal element mounting (7 frames)
#153    SingleChildWidgetElementMixin.mount (package:nested/nested.dart:222:11)
...     Normal element mounting (7 frames)
#160    SingleChildWidgetElementMixin.mount (package:nested/nested.dart:222:11)
...     Normal element mounting (275 frames)
#435    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3817:16)
#436    MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6350:36)
#437    MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6362:32)
...     Normal element mounting (377 frames)
#814    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3817:16)
#815    Element.updateChild (package:flutter/src/widgets/framework.dart:3551:18)
#816    RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1202:16)
#817    RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1171:5)
#818    RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1119:18)
#819    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2597:19)
#820    RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1118:13)
#821    WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:953:7)
#822    WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:933:7)
(elided 11 frames from class _RawReceivePortImpl, class _Timer, dart:async, and dart:async-patch)

引发了另一个异常：每个子项必须恰好布局一次。