d779fc779d
This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/blob/main/docs/releases/Flutter-Cherrypick-Process.md#automatically-creates-a-cherry-pick-request) Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request. ### Issue Link: What is the link to the issue this cherry-pick is addressing? https://github.com/flutter/flutter/issues/168936. ### Changelog Description: Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples Fixes a "Null check operator used on a null value" crash when a scroll view contains a `LayoutBuilder`. ### Impact Description: What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch) The app may crash if there a `LayoutBuilder` child inside of a `RenderSliverMultiBoxAdaptor` (which `SliverList` uses), and that child suddenly becomes offstage and kept-alive. One example is when you have a TextField inside of a LayoutBuilder which is part of a `SliverList`. The app crashes ("Null check operator used on a null value") even in release mode when the user drags so that the text field becomes offscreen. All 3 are popular widgets so I think this combination will not be that uncommon. ### Workaround: Is there a workaround for this issue? There's no known workaround for this issue. You can increase the cache extent to prevent the child from becoming kept-alive but this is not really feasible for long / infinite list as it will go OOM fast. ### Risk: What is the risk level of this cherry-pick? This should be a relatively safe fix. Despite changing an important flag in RenderObject from `RenderObject?` to `bool?`, there's only one change in behavior: in `RenderObject.dropChild`: https://github.com/flutter/flutter/pull/169958/files#diff-4c298197a8aa7831a26eece096c8b8b07773ba1f5376d848ea4ef2924b606e9fL2052 which originally set the flag to `null` for the entire subtree until a new relayout boundary is reached, now it only sets the flag to `null` for the root of the subtree. In release mode, the only place the framework cares about whether the flag is null is [here](https://github.com/flutter/flutter/pull/169958/files#diff-4c298197a8aa7831a26eece096c8b8b07773ba1f5376d848ea4ef2924b606e9fR2342) (everywhere else `null` just means `false`), and that won't actually introduce any correctness issues because if the node originally enters the `if` block in line 2343 it will still enter the `if` block after the patch. Also, the fix was merged a couple weeks ago and I haven't seen bug reports associated with the fix yet. ### Test Coverage: Are you confident that your fix is well-tested by automated tests? ### Validation Steps: What are the steps to validate that this fix works? follow the repro steps in https://github.com/flutter/flutter/issues/168936, or run the test (already checked in in `master`) ```dart testWidgets('LayoutBuilder does not crash when it becomes kept-alive', ( WidgetTester tester, ) async { final FocusNode focusNode = FocusNode(); final TextEditingController controller = TextEditingController(); addTearDown(focusNode.dispose); addTearDown(controller.dispose); final Widget layoutBuilderWithParent = SizedBox( key: GlobalKey(), child: LayoutBuilder( builder: (BuildContext _, BoxConstraints _) { // The text field keeps the widget alive in the SliverList. return EditableText( focusNode: focusNode, backgroundCursorColor: const Color(0xFFFFFFFF), cursorColor: const Color(0xFFFFFFFF), style: const TextStyle(), controller: controller, ); }, ), ); await tester.pumpWidget( Directionality( textDirection: TextDirection.ltr, child: CustomScrollView( slivers: <Widget>[ SliverList.list( addRepaintBoundaries: false, addSemanticIndexes: false, children: <Widget>[const SizedBox(height: 60), layoutBuilderWithParent], ), ], ), ), ); focusNode.requestFocus(); await tester.pumpWidget( Directionality( textDirection: TextDirection.ltr, child: CustomScrollView( slivers: <Widget>[ SliverList.list( addRepaintBoundaries: false, addSemanticIndexes: false, children: <Widget>[const SizedBox(height: 6000), layoutBuilderWithParent], ), ], ), ), ); }); ```
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