[Widget Inspector] Handle null exceptions calling `renderObject` #163642

elliette · 2025-02-19T18:55:15Z

Based on the stacktrace in flutter/devtools#8905:

This call to renderObject can throw a null-exception:

flutter/packages/flutter/lib/src/widgets/framework.dart

Line 3745 in 39b4951

return current.renderObject;
That exception is thrown here:

flutter/packages/flutter/lib/src/widgets/framework.dart

Line 6534 in 39b4951

return _renderObject!;

I've been unable to figure out a way to get into a state that reproduces this. Instead, this PR simply handles the exception and returns null (because we already gracefully handle the case where the renderObject is null.

flutter-dashboard · 2025-02-19T18:55:21Z

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packages/flutter/lib/src/widgets/widget_inspector.dart

kenzieschmoll

LGTM w/ one comment

chunhtai · 2025-03-06T21:00:25Z

packages/flutter/lib/src/widgets/widget_inspector.dart

+  /// If the call to get the render object throws, the result will be null.
+  RenderObject? _renderObjectOrNull(Element element) {
+    try {
+      return element.renderObject;


you can check if this is mounted first that guaranteed to be non null if mounted is true

Thank you for the review!

It looks like this is the mounted getter:

flutter/packages/flutter/lib/src/widgets/framework.dart

Line 3601 in 8876bcc

bool get mounted => _widget != null;

However, it's checking if _widget is non-null, not _renderObject. From what I can tell, _widget is set as part of the createElement step, whereas the _renderObject is set as part of the mount step:

flutter/packages/flutter/lib/src/widgets/framework.dart

Lines 3465 to 3471 in 8876bcc

/// * The framework creates an element by calling [Widget.createElement] on the

/// widget that will be used as the element's initial configuration.

/// * The framework calls [mount] to add the newly created element to the tree

/// at a given slot in a given parent. The [mount] method is responsible for

/// inflating any child widgets and calling [attachRenderObject] as

/// necessary to attach any associated render objects to the render tree.

/// * At this point, the element is considered "active" and might appear on

So I think we would still hit the null-pointer exception when the element has been created but not mounted:

flutter/packages/flutter/lib/src/widgets/framework.dart

Line 6515 in 8876bcc

assert(_renderObject != null, '$runtimeType unmounted');

I might be missing something here though!

looking at the code, I think creating element will synchronouslly mount the element and thus create renderobject. If you find a case it is not, this should be a bug that we need to fix.

flutter/packages/flutter/lib/src/widgets/framework.dart

Line 4534 in 66e910d

final Element newChild = newWidget.createElement();

See line 4534 created the element and mount directly in line 4539

Got it, thanks! Switched to checking if the element is mounted.

Still trying to figure out how best to add tests for this

Added a test case (and confirmed it fails without this change)

chunhtai

LGTM

…ct` (flutter/flutter#163642)

flutter/flutter@6b93cf9...b16430b 2025-03-10 [email protected] [macOS] Enable Impeller by default on macOS. (flutter/flutter#164572) 2025-03-10 [email protected] Roll Packages from 4c5a7ed to 464cea5 (5 revisions) (flutter/flutter#164904) 2025-03-10 [email protected] Roll Skia from f17d37ee0ac6 to 4ac86f17f2d4 (1 revision) (flutter/flutter#164893) 2025-03-10 [email protected] Roll Skia from 0f53870c7449 to f17d37ee0ac6 (1 revision) (flutter/flutter#164887) 2025-03-09 [email protected] Roll Fuchsia Linux SDK from 6tAcm4hdtXPE55GJP... to U-zlyIZrZRbr9I6gv... (flutter/flutter#164868) 2025-03-09 [email protected] Roll Skia from 345dc2d05dcd to 0f53870c7449 (1 revision) (flutter/flutter#164865) 2025-03-08 [email protected] Roll Skia from 916caa2f0102 to 345dc2d05dcd (1 revision) (flutter/flutter#164843) 2025-03-08 [email protected] Roll Fuchsia Linux SDK from ixl5bKWCqsRiYGvps... to 6tAcm4hdtXPE55GJP... (flutter/flutter#164838) 2025-03-08 [email protected] Roll Skia from b29851b2ada6 to 916caa2f0102 (1 revision) (flutter/flutter#164835) 2025-03-08 [email protected] [Impeller] add capability check for extended range formats. (flutter/flutter#164817) 2025-03-08 [email protected] Added calendar delegate to support custom calendar systems (flutter/flutter#161874) 2025-03-08 [email protected] RoundSuperellipse algorithm v3: Ultrawideband heuristic formula (flutter/flutter#164755) 2025-03-08 [email protected] Merge CHANGELOG for 3.29.1 stable release (flutter/flutter#164743) 2025-03-08 [email protected] Add and link to `Infra-Triage.md`. (flutter/flutter#164673) 2025-03-07 [email protected] Roll Skia from cbc7e99d6c2f to b29851b2ada6 (10 revisions) (flutter/flutter#164812) 2025-03-07 [email protected] [Impeller] dont redundantly set stencil reference on vulkan backend. (flutter/flutter#164763) 2025-03-07 [email protected] content-aware-hash experiment update (flutter/flutter#164803) 2025-03-07 [email protected] [Widget Inspector] Handle null exceptions calling `renderObject` (flutter/flutter#163642) 2025-03-07 [email protected] Use Python 3.12 to run the yapf formatter if no lower version is available (flutter/flutter#164807) 2025-03-07 [email protected] Roll gn to 7a8aa3a08a13521336853a28c46537ec04338a2d (flutter/flutter#164806) 2025-03-07 [email protected] [Impeller] Store the TextureGLES cached framebuffer object as a reactor handle (flutter/flutter#164761) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC [email protected],[email protected] on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://issues.skia.org/issues/new?component=1389291&template=1850622 Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

…ct` (flutter/flutter#163642)

flutter/flutter@6b93cf9...b16430b 2025-03-10 [email protected] [macOS] Enable Impeller by default on macOS. (flutter/flutter#164572) 2025-03-10 [email protected] Roll Packages from 4c5a7ed to 464cea5 (5 revisions) (flutter/flutter#164904) 2025-03-10 [email protected] Roll Skia from f17d37ee0ac6 to 4ac86f17f2d4 (1 revision) (flutter/flutter#164893) 2025-03-10 [email protected] Roll Skia from 0f53870c7449 to f17d37ee0ac6 (1 revision) (flutter/flutter#164887) 2025-03-09 [email protected] Roll Fuchsia Linux SDK from 6tAcm4hdtXPE55GJP... to U-zlyIZrZRbr9I6gv... (flutter/flutter#164868) 2025-03-09 [email protected] Roll Skia from 345dc2d05dcd to 0f53870c7449 (1 revision) (flutter/flutter#164865) 2025-03-08 [email protected] Roll Skia from 916caa2f0102 to 345dc2d05dcd (1 revision) (flutter/flutter#164843) 2025-03-08 [email protected] Roll Fuchsia Linux SDK from ixl5bKWCqsRiYGvps... to 6tAcm4hdtXPE55GJP... (flutter/flutter#164838) 2025-03-08 [email protected] Roll Skia from b29851b2ada6 to 916caa2f0102 (1 revision) (flutter/flutter#164835) 2025-03-08 [email protected] [Impeller] add capability check for extended range formats. (flutter/flutter#164817) 2025-03-08 [email protected] Added calendar delegate to support custom calendar systems (flutter/flutter#161874) 2025-03-08 [email protected] RoundSuperellipse algorithm v3: Ultrawideband heuristic formula (flutter/flutter#164755) 2025-03-08 [email protected] Merge CHANGELOG for 3.29.1 stable release (flutter/flutter#164743) 2025-03-08 [email protected] Add and link to `Infra-Triage.md`. (flutter/flutter#164673) 2025-03-07 [email protected] Roll Skia from cbc7e99d6c2f to b29851b2ada6 (10 revisions) (flutter/flutter#164812) 2025-03-07 [email protected] [Impeller] dont redundantly set stencil reference on vulkan backend. (flutter/flutter#164763) 2025-03-07 [email protected] content-aware-hash experiment update (flutter/flutter#164803) 2025-03-07 [email protected] [Widget Inspector] Handle null exceptions calling `renderObject` (flutter/flutter#163642) 2025-03-07 [email protected] Use Python 3.12 to run the yapf formatter if no lower version is available (flutter/flutter#164807) 2025-03-07 [email protected] Roll gn to 7a8aa3a08a13521336853a28c46537ec04338a2d (flutter/flutter#164806) 2025-03-07 [email protected] [Impeller] Store the TextureGLES cached framebuffer object as a reactor handle (flutter/flutter#164761) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC [email protected],[email protected] on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://issues.skia.org/issues/new?component=1389291&template=1850622 Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

…tter#163642) Fixes flutter/devtools#8905 Based on the stacktrace in flutter/devtools#8905: * This call to `renderObject` can throw a null-exception: https://github.com/flutter/flutter/blob/39b4951f8f0bb7a32532ee2f67e83a783b065b58/packages/flutter/lib/src/widgets/framework.dart#L3745 * That exception is thrown here: https://github.com/flutter/flutter/blob/39b4951f8f0bb7a32532ee2f67e83a783b065b58/packages/flutter/lib/src/widgets/framework.dart#L6534 I've been unable to figure out a way to get into a state that reproduces this. Instead, this PR simply handles the exception and returns `null` (because we already gracefully handle the case where the `renderObject` is `null`.

Catch any errors for call to renderObject

32dd256

github-actions bot added the framework flutter/packages/flutter repository. See also f: labels. label Feb 19, 2025

kenzieschmoll reviewed Mar 6, 2025

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elliette added 2 commits March 6, 2025 10:47

Merge branch 'main' into issue-8905

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elliette added 3 commits March 6, 2025 15:21

Check if element is mounted

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Added a test

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Clean up

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Merge branch 'master' into issue-8905

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Merged via the queue into flutter:master with commit b7bea22 Mar 7, 2025
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	/// * The framework creates an element by calling [Widget.createElement] on the
	/// widget that will be used as the element's initial configuration.
	/// * The framework calls [mount] to add the newly created element to the tree
	/// at a given slot in a given parent. The [mount] method is responsible for
	/// inflating any child widgets and calling [attachRenderObject] as
	/// necessary to attach any associated render objects to the render tree.
	/// * At this point, the element is considered "active" and might appear on

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[Widget Inspector] Handle null exceptions calling `renderObject` #163642

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