Flutter项目更新后Windows/Chrome出现Syncfusion组件空安全类型错误

问题描述 #

项目此前运行正常，Flutter更新后，Windows和Chrome平台均出现4个可空类型不匹配错误，涉及syncfusion_flutter_calendar-20.4.54和syncfusion_flutter_datepicker-20.4.54两个组件。

Windows平台构建错误 #

构建时出现以下4个类型不匹配错误，最终导致MSB8066构建失败：

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_calendar-20.4.54/lib/src/calendar/sfcalendar.dart(10776,71): error G44692867: A value of type 'RenderAbstractViewport?' can't be returned from a function with return type 'RenderAbstractViewport' because 'RenderAbstractViewport?' is nullable and 'RenderAbstractViewport' isn't. [C:\Users\YuriVittori\Documents\MeusProjetos\Project_002_Italian_Citizenship\build\windows\flutter\flutter_assemble.vcxproj]

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart(7596,37): error GC2F972A8: The argument type 'ScrollableState?' can't be assigned to the parameter type 'ScrollableState' because 'ScrollableState?' is nullable and 'ScrollableState' isn't. [C:\Users\YuriVittori\Documents\MeusProjetos\Project_002_Italian_Citizenship\build\windows\flutter\flutter_assemble.vcxproj]

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart(7611,40): error G44692867: A value of type 'ScrollableState?' can't be assigned to a variable of type 'ScrollableState' because 'ScrollableState?' is nullable and 'ScrollableState' isn't. [C:\Users\YuriVittori\Documents\MeusProjetos\Project_002_Italian_Citizenship\build\windows\flutter\flutter_assemble.vcxproj]

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart(7665,71): error G44692867: A value of type 'RenderAbstractViewport?' can't be returned from a function with return type 'RenderAbstractViewport' because 'RenderAbstractViewport?' is nullable and 'RenderAbstractViewport' isn't. [C:\Users\YuriVittori\Documents\MeusProjetos\Project_002_Italian_Citizenship\build\windows\flutter\flutter_assemble.vcxproj]

Chrome平台调试错误 #

启动调试时出现完全相同的4个错误：

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_calendar-20.4.54/lib/src/calendar/sfcalendar.dart:10776:71: Error: A value of type 'RenderAbstractViewport?' can't be returned from a function with return type 'RenderAbstractViewport' because 'RenderAbstractViewport?' is nullable and 'RenderAbstractViewport' isn't.

• 'RenderAbstractViewport' is from 'package:flutter/src/rendering/viewport.dart' ('../../../Downloads/flutter/packages/flutter/lib/src/rendering/viewport.dart').
  RenderAbstractViewport get _stackViewPort => RenderAbstractViewport.of(this);
  ^

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart:7596:37: Error: The argument type 'ScrollableState?' can't be assigned to the parameter type 'ScrollableState' because 'ScrollableState?' is nullable and 'ScrollableState' isn't.

• 'ScrollableState' is from 'package:flutter/src/widgets/scrollable.dart' ('../../../Downloads/flutter/packages/flutter/lib/src/widgets/scrollable.dart').
  scrollableState: Scrollable.of(context),
  ^

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart:7611:40: Error: A value of type 'ScrollableState?' can't be assigned to a variable of type 'ScrollableState' because 'ScrollableState?' is nullable and 'ScrollableState' isn't.

• 'ScrollableState' is from 'package:flutter/src/widgets/scrollable.dart' ('../../../Downloads/flutter/packages/flutter/lib/src/widgets/scrollable.dart').
  ..scrollableState = Scrollable.of(context)
  ^

../../../Downloads/flutter/.pub-cache/hosted/pub.dartlang.org/syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart:7665:71: Error: A value of type 'RenderAbstractViewport?' can't be returned from a function with return type 'RenderAbstractViewport' because 'RenderAbstractViewport?' is nullable and 'RenderAbstractViewport' isn't.

• 'RenderAbstractViewport' is from 'package:flutter/src/rendering/viewport.dart' ('../../../Downloads/flutter/packages/flutter/lib/src/rendering/viewport.dart').
  RenderAbstractViewport get _stackViewPort => RenderAbstractViewport.of(this);
  ^

解决办法 #

1. 更新Syncfusion组件到兼容版本 #

你使用的syncfusion_flutter_calendar和syncfusion_flutter_datepicker版本20.4.54为旧版，不兼容更新后的Flutter空安全API。直接修改pubspec.yaml中组件版本为最新稳定版：

dependencies:
  syncfusion_flutter_calendar: ^latest_version
  syncfusion_flutter_datepicker: ^latest_version

执行命令更新依赖：

flutter pub get

Syncfusion后续版本已修复该空类型不匹配问题，这是最优解决方案。

2. 临时源码修改（应急用） #

若暂时无法升级组件，可手动修改pub缓存中的源码：

• 定位到syncfusion_flutter_calendar-20.4.54/lib/src/calendar/sfcalendar.dart第10776行，改为：

  RenderAbstractViewport get _stackViewPort => RenderAbstractViewport.of(this)!;
  

• 定位到syncfusion_flutter_datepicker-20.4.54/lib/src/date_picker/date_picker.dart对应行：
  • 7596行：scrollableState: Scrollable.of(context)!
  • 7611行：..scrollableState = Scrollable.of(context)!
  • 7665行：RenderAbstractViewport get _stackViewPort => RenderAbstractViewport.of(this)!;
    注意：该修改会在下次flutter pub get时被覆盖，不建议长期使用。

3. 回退Flutter版本（临时方案） #

若不想升级组件，可回退到之前能正常运行的Flutter版本：

flutter downgrade <your_old_flutter_version>

此方式会限制新特性使用，仅作为临时应急方案。

内容的提问来源于stack exchange，提问作者Duas Vias