错误内容如下：

======== Exception caught by widgets library =======================================================

The following assertion was thrown building _CupertinoTextSelectionControlsToolbar(dirty, dependencies: [MediaQuery, _LocalizationsScope-[GlobalKey#1fa5b]], state: _CupertinoTextSelectionControlsToolbarState#23a43):

No CupertinoLocalizations found.

_CupertinoTextSelectionControlsToolbar widgets require CupertinoLocalizations to be provided by a Localizations widget ancestor.

The cupertino library uses Localizations to generate messages, labels, and abbreviations.

To introduce a CupertinoLocalizations, either use a CupertinoApp at the root of your application to include them automatically, or add a Localization widget with a CupertinoLocalizations delegate.

The specific widget that could not find a CupertinoLocalizations ancestor was: _CupertinoTextSelectionControlsToolbar

  dirty

  dependencies: [MediaQuery, _LocalizationsScope-[GlobalKey#1fa5b]]

  state: _CupertinoTextSelectionControlsToolbarState#23a43

The ancestors of this widget were:

  : CupertinoApp

    state: _CupertinoAppState#5e387

  : MyApp

  : ChangeNotifierProvider<KitUser>

    value: <not yet loaded>

  : MultiProvider

  ...

The relevant error-causing widget was:

  CupertinoApp CupertinoApp:file:///Users/jarvis/Desktop/Flutter-workspace/flutter_oa/code/oa/lib/main.dart:70:14

When the exception was thrown, this was the stack:

#0      debugCheckHasCupertinoLocalizations.<anonymous closure> (package:flutter/src/cupertino/debug.dart:23:7)

#1      debugCheckHasCupertinoLocalizations (package:flutter/src/cupertino/debug.dart:43:4)

#2      CupertinoLocalizations.of (package:flutter/src/cupertino/localizations.dart:262:12)

#3      _CupertinoTextSelectionControlsToolbarState.build (package:flutter/src/cupertino/text_selection.dart:135:73)

#4      StatefulElement.build (package:flutter/src/widgets/framework.dart:4782:27)

#5      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4665:15)

#6      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4840:11)

#7      Element.rebuild (package:flutter/src/widgets/framework.dart:4355:5)

#8      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4643:5)

#9      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4831:11)

#10    ComponentElement.mount (package:flutter/src/widgets/framework.dart:4638:5)

...    Normal element mounting (39 frames)

#49    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3673:14)

#50    MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6333:36)

#51    Element.updateChild (package:flutter/src/widgets/framework.dart:3425:18)

#52    RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5758:32)

#53    MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6356:17)

#54    Element.updateChild (package:flutter/src/widgets/framework.dart:3412:15)

#55    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4690:16)

#56    StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4840:11)

#57    Element.rebuild (package:flutter/src/widgets/framework.dart:4355:5)

#58    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2620:33)

#59    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:882:21)

#60    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:319:5)

#61    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1143:15)

#62    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1080:9)

#63    SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:996:5)

#67    _invoke (dart:ui/hooks.dart:166:10)

#68    PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:270:5)

#69    _drawFrame (dart:ui/hooks.dart:129:31)

(elided 3 frames from dart:async)

====================================================================================================

从上面的错误信息中的一句：

To introduce a CupertinoLocalizations, either use a CupertinoApp at the root of your application to include them automatically, or add a Localization widget with a CupertinoLocalizations delegate. 

大致的意思是需要引用‘CupertinoLocalizations’这么个东西，然后用’CupertinoApp‘这个东西包裹，那么‘CupertinoLocalizations’需要放在那里呢，哎，就是放在语言代理的‘localizationsDelegates’位置下，

当然，不能直接使用‘CupertinoLocalizations’，不然也是报错，这里需要使用的是‘GlobalCupertinoLocalizations.delegate,’，这个代理，我这里是区分了iOS和android设备平台。

具体可参考下图：