构建函数返回空Flutter

Question

这是我的问题，我不知道如何解决。请帮助我

该项目的Github：here

Launching lib/main.dart on iPhone SE (2nd generation) in debug mode...
Running Xcode build...
Xcode build done.                                           22.0s
Configuring the default Firebase app...
Configured the default Firebase app __FIRAPP_DEFAULT.
Waiting for iPhone SE (2nd generation) to report its views...
Debug service listening on ws://127.0.0.1:54630/tFUTjcfl640=/ws
Syncing files to device iPhone SE (2nd generation)...
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building StreamBuilder<CalendarDetailModel>(dirty, state:
flutter: _StreamBuilderBaseState<CalendarDetailModel, AsyncSnapshot<CalendarDetailModel>>#836df):
flutter: A build function returned null.
flutter: The offending widget is:
flutter:   StreamBuilder<CalendarDetailModel>
flutter: Build functions must never return null.
flutter: To return an empty space that causes the building widget to fill available room, return
flutter: "Container()". To return an empty space that takes as little room as possible, return
flutter: "Container(width: 0.0, height: 0.0)".
flutter:
flutter: The relevant error-causing widget was:
flutter:   StreamBuilder<CalendarDetailModel>
flutter:   file:///Users/lethanhtu/Desktop/firebase_authentication/lib/ui/calendar_detail.dart:54:20
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0      debugWidgetBuilderValue.<anonymous closure> (package:flutter/src/widgets/debug.dart:276:7)
flutter: #1      debugWidgetBuilderValue (package:flutter/src/widgets/debug.dart:297:4)
flutter: #2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4516:7)
flutter: #3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4684:11)
flutter: #4      Element.rebuild (package:flutter/src/widgets/framework.dart:4227:5)
flutter: #5      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4490:5)
flutter: #6      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4675:11)
flutter: #7      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4485:5)
flutter: ...     Normal element mounting (39 frames)
flutter: #46     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
flutter: #47     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
flutter: #48     SliverMultiBoxAdaptorElement.updateChild (package:flutter/src/widgets/sliver.dart:1162:36)
flutter: #49     SliverMultiBoxAdaptorElement.createChild.<anonymous closure> (package:flutter/src/widgets/sliver.dart:1147:20)
flutter: #50     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2612:19)
flutter: #51     SliverMultiBoxAdaptorElement.createChild (package:flutter/src/widgets/sliver.dart:1140:11)
flutter: #52     RenderSliverMultiBoxAdaptor._createOrObtainChild.<anonymous closure> (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:354:23)
flutter: #53     RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1866:58)
flutter: #54     PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:918:15)
flutter: #55     RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1866:13)
flutter: #56     RenderSliverMultiBoxAdaptor._createOrObtainChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:343:5)
flutter: #57     RenderSliverMultiBoxAdaptor.addInitialChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:427:5)
flutter: #58     RenderSliverList.performLayout (package:flutter/src/rendering/sliver_list.dart:79:12)
flutter: #59     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #60     RenderSliverEdgeInsetsPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:135:11)
flutter: #61     RenderSliverPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:375:11)
flutter: #62     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #63     RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:452:13)
flutter: #64     RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1443:12)
flutter: #65     RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1352:20)
flutter: #66     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #67     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #68     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #69     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #70     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #71     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #72     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #73     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #74     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #75     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #76     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #77     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #78     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #79     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #80     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #81     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #82     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #83     RenderPadding.performLayout (package:flutter/src/rendering/shifted_box.dart:207:11)
flutter: #84     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #85     MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:171:11)
flutter: #86     _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:480:7)
flutter: #87     MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:240:7)
flutter: #88     RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:399:14)
flutter: #89     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #90     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #91     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #92     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #93     _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1267:11)
flutter: #94     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #95     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #96     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #97     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #98     RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #99     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #100    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #101    RenderStack.performLayout (package:flutter/src/rendering/stack.dart:556:15)
flutter: #102    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #103    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #104    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #105    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #106    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #107    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #108    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #109    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #110    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #111    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #112    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #113    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #114    RenderOffstage.performLayout (package:flutter/src/rendering/proxy_box.dart:3219:13)
flutter: #115    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #116    _RenderTheatre.performLayout (package:flutter/src/widgets/overlay.dart:700:15)
flutter: #117    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #118    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #119    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #120    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #121    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #122    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #123    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #124    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #125    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #126    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:113:13)
flutter: #127    RenderObject.layout (package:flutter/src/rendering/object.dart:1767:7)
flutter: #128    RenderView.performLayout (package:flutter/src/rendering/view.dart:167:13)
flutter: #129    RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1630:7)
flutter: #130    PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:887:18)
flutter: #131    RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:402:19)
flutter: #132    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:865:13)
flutter: #133    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:284:5)
flutter: #134    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113:15)
flutter: #135    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1052:9)
flutter: #136    SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:861:7)
flutter: (elided 11 frames from class _RawReceivePortImpl, class _Timer, dart:async, and dart:async-patch)
flutter:
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════
flutter: Status code is 200
[VERBOSE-2:ui_dart_state.cc(166)] Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'Map<String, String>'
#0      new CalendarDetailModel (package:firebaseauthentication/models/calendar_detail_model.dart:26:40)
#1      CalendarDetailRepository.fetchRoom.<anonymous closure> (package:firebaseauthentication/resources/calendar_detail_repository.dart:38:23)
#2      List.forEach (dart:core-patch/growable_array.dart:289:8)
#3      CalendarDetailRepository.fetchRoom (package:firebaseauthentication/resources/calendar_detail_repository.dart:35:19)
<asynchronous suspension>
#4      CalendarDetailBloc.loadRoom (package:firebaseauthentication/blocs/calendar_detail_bloc.dart:16:36)
#5      CalendarDetailsState.initState (package:firebaseauthentication/ui/calendar_detail.dart:35:10)
#6      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4649:58)
#7      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4485:5)
#8      Element.inflateWidget (package:flutter/<…>

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown building StreamBuilder<CalendarDetailModel>(dirty, state: _StreamBuilderBaseState<CalendarDetailModel, AsyncSnapshot<CalendarDetailModel>>#836df):
A build function returned null.

The offending widget is: StreamBuilder<CalendarDetailModel>
Build functions must never return null.

To return an empty space that causes the building widget to fill available room, return "Container()". To return an empty space that takes as little room as possible, return "Container(width: 0.0, height: 0.0)".

The relevant error-causing widget was: 
  StreamBuilder<CalendarDetailModel> file:///Users/lethanhtu/Desktop/firebase_authentication/lib/ui/calendar_detail.dart:54:20
When the exception was thrown, this was the stack: 
#0      debugWidgetBuilderValue.<anonymous closure> (package:flutter/src/widgets/debug.dart:276:7)
#1      debugWidgetBuilderValue (package:flutter/src/widgets/debug.dart:297:4)
#2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4516:7)
#3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4684:11)
#4      Element.rebuild (package:flutter/src/widgets/framework.dart:4227:5)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

Answer 1

有关问题所在的错误非常清楚。在您StreamBuilder中，您不肯返回任何东西。您有：

builder: (context, snapshot) {
  if (snapshot.hasError) {
    return CircularProgressIndicator();
  } else if (snapshot.hasData) {
    ...
  }

仅在出现错误或数据可用时返回某些内容。但是，并不总是有可用数据或错误，因此，当这两个条件都不成立时，您必须返回一些信息。

只需在这些return CircularProgressIndicator();块之外或if-else语句中添加else就可以解决您的问题。

例如。

builder: (context, snapshot) {
  if (snapshot.hasError) {
    return CircularProgressIndicator();
  } else if (snapshot.hasData) {
    ...
  }
  else {
    return CircularProgressIndicator();
  }

按照这些错误提供的说明进行操作可以节省大量时间，并且无需在SO上询问。