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[widget previews][Windows] Sometimes the widget previewer fails to hot reload and then crashes after restarting the process #192134

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#192325
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[widget previews][Windows] Sometimes the widget previewer fails to hot reload and then crashes after restarting the process#192134
#192325
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bkonyi
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P1High-priority issues at the top of the work lista: widget previewsIssues related to the Widget Previewer toolingteam-toolOwned by Flutter Tool teamtoolAffects the "flutter" command-line tool. See also t: labels.triaged-toolTriaged by Flutter Tool team

Description

@navaronbracke
navaronbracke
opened on Sep 1, 2026

Steps to reproduce

  1. Given a project that has a widget preview function defined
  2. flutter widget-preview start in the terminal
  3. Wait for the browser to open the preview.
  4. At some point after a few hot reloads, hot reload seems to get stuck? (it does not complete after a few seconds, which it did in earlier reloads)
  5. Terminate the batch job in the terminal
  6. flutter widget-preview start in the terminal
  7. "Null check operator used on a null value" appears
  8. A bit later, the process prints a stack trace like below

Expected results

The widget preview hot reloads properly, or at least does not crash after terminating and restarting the process a second time.

Actual results

The preview process eventually crashes with

Unhandled exception:
Null check operator used on a null value
#0      FrontendCompiler.compileExpressionToJs (package:frontend_server/frontend_server.dart:1423)
#1      processJsonInput (package:frontend_server/frontend_server.dart:2092)
#2      listenAndCompile.<anonymous closure> (package:frontend_server/frontend_server.dart:1975)
#3      _RootZone.runUnaryGuarded (dart:async/zone.dart:963)
#4      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:381)
#5      _BufferingStreamSubscription._add (dart:async/stream_impl.dart:312)
#6      _SinkTransformerStreamSubscription._add (dart:async/stream_transformers.dart:67)
#7      _EventSinkWrapper.add (dart:async/stream_transformers.dart:13)
#8      _StringAdapterSink.add (dart:convert/string_conversion.dart:231)
#9      _LineSplitterSink._addLines (dart:convert/line_splitter.dart:164)
#10     _LineSplitterSink.addSlice (dart:convert/line_splitter.dart:131)
#11     StringConversionSink.add (dart:convert/string_conversion.dart:40)
#12     _SinkTransformerStreamSubscription._handleData (dart:async/stream_transformers.dart:115)
#13     _RootZone.runUnaryGuarded (dart:async/zone.dart:963)
#14     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:381)
#15     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:312)
#16     _SinkTransformerStreamSubscription._add (dart:async/stream_transformers.dart:67)
#17     _EventSinkWrapper.add (dart:async/stream_transformers.dart:13)
#18     _StringAdapterSink.add (dart:convert/string_conversion.dart:231)
#19     _StringAdapterSink.addSlice (dart:convert/string_conversion.dart:236)
#20     _Utf8ConversionSink.addSlice (dart:convert/string_conversion.dart:316)
#21     _Utf8ConversionSink.add (dart:convert/string_conversion.dart:309)
#22     _ConverterStreamEventSink.add (dart:convert/chunked_conversion.dart:70)
#23     _SinkTransformerStreamSubscription._handleData (dart:async/stream_transformers.dart:115)
#24     _RootZone.runUnaryGuarded (dart:async/zone.dart:963)
#25     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:381)
#26     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:312)
#27     _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:798)
#28     _StreamController._add (dart:async/stream_controller.dart:663)
#29     _StreamController.add (dart:async/stream_controller.dart:618)
#30     _Socket._onData (dart:io-patch/socket_patch.dart:2906)
#31     _RootZone.runUnaryGuarded (dart:async/zone.dart:963)
#32     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:381)
#33     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:312)
#34     _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:798)
#35     _StreamController._add (dart:async/stream_controller.dart:663)
#36     _StreamController.add (dart:async/stream_controller.dart:618)
#37     new _RawSocket.<anonymous closure> (dart:io-patch/socket_patch.dart:2344)
#38     _NativeSocket.issueReadEvent.issue (dart:io-patch/socket_patch.dart:1679)
#39     _microtaskLoop (dart:async/schedule_microtask.dart:40)
#40     _startMicrotaskLoop (dart:async/schedule_microtask.dart:49)
#41     _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:127)
#42     _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:193)
The Dart compiler exited unexpectedly.
ChromeProxyService: Failed to evaluate expression 'true': InternalError: frontend server failed to compile '() { return true; }'.
Failed to evaluate expression 'true': InternalError: frontend server failed to compile '() { return true; }'.

Code sample

Code sample 
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Screenshots or Video

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Logs

widget_preview_log.txt

Flutter Doctor output

Doctor output 
[√] Flutter (Channel stable, 3.47.2, on Microsoft Windows [Version 10.0.26200.9168], locale nl-BE) [247ms]
    • Flutter version 3.47.2 on channel stable at C:\Users\brack\Documents\flutter
    • Upstream repository [email protected]:navaronbracke/flutter.git
    • FLUTTER_GIT_URL = [email protected]:navaronbracke/flutter.git
    • Framework revision d3b14c8769 (6 days ago), 2026-08-26 16:07:51 -0700
    • Engine revision a804b26164
    • Dart version 3.13.2
    • DevTools version 2.60.0
    • Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets, enable-record-use, enable-swift-package-manager,
      omit-legacy-version-file, enable-lldb-debugging, enable-uiscene-migration

[√] Windows Version (11 Home 64-bit, 25H2, 2009) [729ms]

[X] Android toolchain - develop for Android devices [107ms]
    X Unable to locate Android SDK.
      Install Android Studio from: https://developer.android.com/studio/index.html
      On first launch it will assist you in installing the Android SDK components.
      (or visit https://flutter.dev/to/windows-android-setup for detailed instructions).
      If the Android SDK has been installed to a custom location, please use
      `flutter config --android-sdk` to update to that location.


[X] Chrome - develop for the web (Cannot find Chrome executable at .\Google\Chrome\Application\chrome.exe) [96ms]
    ! Cannot find Chrome. Try setting CHROME_EXECUTABLE to a Chrome executable.

[√] Visual Studio - develop Windows apps (Visual Studio Community 2026 18.5.0) [95ms]
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\18\Community
    • Visual Studio Community 2026 version 18.5.11709.299
    • Windows 10 SDK version 10.0.26100.0

[√] Connected device (2 available) [49ms]
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.26200.9168]
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 152.0.4191.53

[√] Network resources [504ms]
    • All expected network resources are available.

! Doctor found issues in 2 categories.

Activity

  1. puneetkukreja98
    added
    in triagePresently being triaged by the triage team
    on Sep 2, 2026

  2. puneetkukreja98 commented on Sep 2, 2026

    @puneetkukreja98
    puneetkukreja98
    on Sep 2, 2026
    Contributor

    @navaronbracke Thanks for reporting this, routing this to team-tool for further investigation.

  3. puneetkukreja98
    added
    toolAffects the "flutter" command-line tool. See also t: labels.
    team-toolOwned by Flutter Tool team
    a: widget previewsIssues related to the Widget Previewer tooling
    and removed
    in triagePresently being triaged by the triage team
    on Sep 2, 2026
  4. bkonyi
    self-assigned this
    on Sep 4, 2026
  5. bkonyi
    added
    P1High-priority issues at the top of the work list
    triaged-toolTriaged by Flutter Tool team
    on Sep 4, 2026
  6. auto-submit
    closed this as completedin #192325on Oct 3, 2026
  7. ahmedsameha1
    added a commit that references this issue on Oct 3, 2026

    [flutter_tools] serialize and gate widget preview hot reloads (flutte…

    243d5f0

  8. github-actions commented on Oct 17, 2026

    @github-actions
    github-actionsBot
    on Oct 17, 2026 – with GitHub Actions

    This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.

  9. github-actions
    locked as resolved and limited conversation to collaborators on Oct 17, 2026
  10. suryaKommana2662
    added a commit that references this issue on Oct 29, 2026

    [flutter_tools] serialize and gate widget preview hot reloads (flutte…

    e770220
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P1High-priority issues at the top of the work lista: widget previewsIssues related to the Widget Previewer toolingteam-toolOwned by Flutter Tool teamtoolAffects the "flutter" command-line tool. See also t: labels.triaged-toolTriaged by Flutter Tool team

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