palette_generator Bad state: Future already completed

I'm working with a package which is called palette_generator. I see an error when I run my app with a few line codes to use the package. I thought that the error might have caused by my code, but I couldn't figure it out. Finally, I tried to edit the source code of this package. Then, it worked without any error. However, I am not sure that the solution that I found is consistent. I'd like to share the issue and my solution in order to help other people and contribute the package.

# The Issue

The error appears when I try to call the following function inside `initState()`. The function is the same with the one which is shown as an example in its documentation:

```dart
PaletteGenerator _paletteGenerator;

void initState() {
    super.initState();
    _updatePaletteGenerator(region);
}

Future<void> _updatePaletteGenerator(Rect newRegion) async {
    paletteGenerator = await PaletteGenerator.fromImageProvider(
      widget.image,
      size: widget.imageSize,
      region: newRegion,
      maximumColorCount: 20,
    );
    setState(() {});
}
```
The error looks this:

```
flutter: ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
flutter: The following StateError was thrown by an image listener:
flutter: Bad state: Future already completed
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #1      PaletteGenerator.fromImageProvider.imageListener (package:palette_generator/palette_generator.dart:188:22)
flutter: #2      ImageStreamCompleter.setImage (package:flutter/src/painting/image_stream.dart:282:17)
flutter: #3      MultiFrameImageStreamCompleter._emitFrame (package:flutter/src/painting/image_stream.dart:519:5)
flutter: #4      MultiFrameImageStreamCompleter._decodeNextFrameAndSchedule (package:flutter/src/painting/image_stream.dart:512:7)
flutter: <asynchronous suspension>
flutter: #5      MultiFrameImageStreamCompleter.addListener (package:flutter/src/painting/image_stream.dart:528:7)
flutter: #6      ImageStream.addListener (package:flutter/src/painting/image_stream.dart:156:25)
flutter: #7      PaletteGenerator.fromImageProvider (package:palette_generator/palette_generator.dart:200:12)
flutter: <asynchronous suspension>
flutter: #8      _TestScreenState._updatePaletteGenerator (package:test_app/screens/test_screen.dart:28:22)
flutter: #9      _TestScreenState.initState (package:test_app/screens/test_screen.dart:15:5)
flutter: #10     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3763:58)
flutter: #11     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #12     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #13     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #14     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #15     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #16     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #17     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #18     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #19     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #20     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #21     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #22     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #23     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #24     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #25     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #26     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #27     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #28     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4771:32)
flutter: #29     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #30     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #31     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #32     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #33     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #34     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #35     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #36     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #37     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #38     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #39     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #40     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #41     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #42     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #43     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #44     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #45     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #46     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #47     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #48     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #49     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #50     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #51     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #52     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #53     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #54     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #55     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #56     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #57     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #58     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #59     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #60     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #61     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #62     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #63     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #64     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #65     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #66     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #67     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #68     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #69     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #70     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #71     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #72     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #73     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #74     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #75     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #76     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #77     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #78     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #79     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #80     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #81     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #82     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #83     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #84     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #85     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #86     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #87     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #88     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #89     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #90     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #91     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #92     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #93     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #94     Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #95     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #96     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #97     ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #98     Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #99     Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #100    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #101    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #102    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #103    ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #104    Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #105    Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #106    SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:4666:14)
flutter: #107    Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #108    Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #109    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #110    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #111    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #112    ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #113    Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #114    Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #115    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #116    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #117    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #118    StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #119    ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #120    Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #121    Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #122    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #123    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #124    ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3634:5)
flutter: #125    StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3781:11)
flutter: #126    ComponentElement.mount (package:flutter/src/widgets/framework.dart:3629:5)
flutter: #127    Element.inflateWidget (package:flutter/src/widgets/framework.dart:2919:14)
flutter: #128    Element.updateChild (package:flutter/src/widgets/framework.dart:2722:12)
flutter: #129    RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4449:32)
flutter: #130    MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4781:17)
flutter: #131    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #132    _TheatreElement.update (package:flutter/src/widgets/overlay.dart:507:16)
flutter: #133    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #134    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #135    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #136    StatefulElement.update (package:flutter/src/widgets/framework.dart:3811:5)
flutter: #137    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #138    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #139    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #140    ProxyElement.update (package:flutter/src/widgets/framework.dart:3921:5)
flutter: #141    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #142    SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4673:14)
flutter: #143    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #144    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #145    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #146    StatefulElement.update (package:flutter/src/widgets/framework.dart:3811:5)
flutter: #147    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #148    SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4673:14)
flutter: #149    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #150    SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4673:14)
flutter: #151    Element.updateChild (package:flutter/src/widgets/framework.dart:2711:15)
flutter: #152    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3665:16)
flutter: #153    Element.rebuild (package:flutter/src/widgets/framework.dart:3507:5)
flutter: #154    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2255:33)
flutter: #155    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:653:20)
flutter: #156    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:208:5)
flutter: #157    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)
flutter: #158    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)
flutter: #159    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:842:5)
flutter: #160    _invoke (dart:ui/hooks.dart:128:13)
flutter: #161    _drawFrame (dart:ui/hooks.dart:117:3)
flutter: (elided one frame from package dart:async)
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════
```

# The Solution

I replaced this code in `package:palette_generator/palette_generator.dart:188:22`:

```dart
imageCompleter.complete(info.image);
```

With this:

```dart
if (!imageCompleter.isCompleted) {
    imageCompleter.complete(info.image);
}
```




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palette_generator Bad state: Future already completed #22754

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palette_generator Bad state: Future already completed #22754

Description

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Metadata

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