"Null check operator used on a null value" while building SfLegend

[KKshitiz]

This is the exception I'm getting while using the Legend widget in SfCartesianChart
The error stack is as follows:

════════ Exception caught by widgets library ═══════════════════════════════════
The following _CastError was thrown building SfLegend(dirty, dependencies: [_LocalizationsScope-[GlobalKey#265f5], _InheritedTheme], state: _SfLegendState#2a76d):
Null check operator used on a null value

The relevant error-causing widget was
SfCartesianChart
lib\…\widgets\graph_tab_contents.dart:73
When the exception was thrown, this was the stack
#0      _SfLegendState.build
package:syncfusion_flutter_core/…/legend/legend.dart:837
#1      StatefulElement.build
package:flutter/…/widgets/framework.dart:4782
#2      ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4665
#3      StatefulElement.performRebuild
package:flutter/…/widgets/framework.dart:4840
#4      Element.rebuild
package:flutter/…/widgets/framework.dart:4355
#5      ComponentElement._firstBuild
package:flutter/…/widgets/framework.dart:4643
#6      StatefulElement._firstBuild
package:flutter/…/widgets/framework.dart:4831
#7      ComponentElement.mount
package:flutter/…/widgets/framework.dart:4638
#8      Element.inflateWidget
package:flutter/…/widgets/framework.dart:3673
#9      Element.updateChild
package:flutter/…/widgets/framework.dart:3425
#10     _LayoutBuilderElement._layout.layoutCallback
package:flutter/…/widgets/layout_builder.dart:137
#11     BuildOwner.buildScope
package:flutter/…/widgets/framework.dart:2573
#12     _LayoutBuilderElement._layout
package:flutter/…/widgets/layout_builder.dart:154
#13     RenderObject.invokeLayoutCallback.<anonymous closure>
package:flutter/…/rendering/object.dart:1968
#14     PipelineOwner._enableMutationsToDirtySubtrees
package:flutter/…/rendering/object.dart:916
#15     RenderObject.invokeLayoutCallback
package:flutter/…/rendering/object.dart:1968
#16     RenderConstrainedLayoutBuilder.rebuildIfNecessary
package:flutter/…/widgets/layout_builder.dart:228
#17     _RenderLayoutBuilder.performLayout
package:flutter/…/widgets/layout_builder.dart:363
#18     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#19     ChildLayoutHelper.layoutChild
package:flutter/…/rendering/layout_helper.dart:56
#20     RenderFlex._computeSizes
package:flutter/…/rendering/flex.dart:896
#21     RenderFlex.performLayout
package:flutter/…/rendering/flex.dart:931
#22     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#23     RenderPadding.performLayout
package:flutter/…/rendering/shifted_box.dart:233
#24     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#25     RenderPadding.performLayout
package:flutter/…/rendering/shifted_box.dart:233
#26     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#27     RenderProxyBoxMixin.performLayout
package:flutter/…/rendering/proxy_box.dart:116
#28     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#29     _ChartContainerBox.performLayout
package:syncfusion_flutter_charts/…/common/common.dart:39
#30     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#31     RenderProxyBoxMixin.performLayout
package:flutter/…/rendering/proxy_box.dart:116
#32     RenderObject.layout
package:flutter/…/rendering/object.dart:1858
#33     ChildLayoutHelper.layoutChild
package:flutter/…/rendering/layout_helper.dart:56
#34     RenderFlex._computeSizes
package:flutter/…/rendering/flex.dart:829
#35     RenderFlex.performLayout
package:flutter/…/rendering/flex.dart:931
#36     RenderObject.layout
package:flutter/…/rendering/object.dart:1858

The debugger throws the exception on line 835 of the legend.dart file:

_textStyle = themeData.textTheme.caption!
         .copyWith(
             color: themeData.textTheme.caption!.color!.withOpacity(0.87))
         .merge(widget.textStyle);

I'm using the latest version of the package:
syncfusion_flutter_charts: ^19.3.53

Also, I'm on the null safe version of Flutter:

environment:
  sdk: ">=2.12.0 <3.0.0"

This is the output of "flutter doctor -v":

$ flutter doctor -v
[√] Flutter (Channel stable, 2.5.3, on Microsoft Windows [Version 10.0.22000.346], locale en-US)
    • Flutter version 2.5.3 at C:\src\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 18116933e7 (4 weeks ago), 2021-10-15 10:46:35 -0700
    • Engine revision d3ea636dc5
    • Dart version 2.14.4

[√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at C:\Users\skili\AppData\Local\Android\sdk
    • Platform android-31, build-tools 31.0.0
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Professional 2019 16.11.6)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional
    • Visual Studio Professional 2019 version 16.11.31829.152
    • Windows 10 SDK version 10.0.19041.0

[√] Android Studio (version 2020.3)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)

[√] VS Code (version 1.62.2)
    • VS Code at C:\Users\skili\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.28.0

[√] Connected device (5 available)
    • Redmi Note 9 Pro (mobile)          • 1a4fb08a      • android-arm64  • Android 11 (API 30)     
    • Android SDK built for x86 (mobile) • emulator-5554 • android-x86    • Android 8.0.0 (API 26)  
      (emulator)
    • Windows (desktop)                  • windows       • windows-x64    • Microsoft Windows
      [Version 10.0.22000.346]
    • Chrome (web)                       • chrome        • web-javascript • Google Chrome
      95.0.4638.69
    • Edge (web)                         • edge          • web-javascript • Microsoft Edge
      95.0.1020.44

• No issues found!

[KKshitiz]

I've tried cleaning the pub cache and making sure it picks up the latest version of the package but no effect. Also, when I set Legend visibility to false or completely remove the Legend from SfCartesianChart, it works fine!

[Yuvaraj-Gajaraj]

Hi @KKshitiz

Greetings from Syncfusion. We have analyzed your query, and we have tried to reproduce the reported issue regarding getting a null exception when building the SfCartesianChart with legend, but it is not reproduced at our end. We have also attached the sample below which Is used for testing.

Sample:
f170367.zip

Since we are not aware of your exact scenario on which the issue is replicating, kindly provide your code or try to replicate it with the attached sample. This will be helpful in further analysis and provide you the solution sooner.

Regards,
Yuvaraj.

[KKshitiz]

Thanks @Yuvaraj-Gajaraj for the help. While digging down a bit deeper, I tested it without defining the theme for the Material App and it worked! Later realized that the package was trying to tell the same thing:

_textStyle = themeData.textTheme.caption!
         .copyWith(
             color: themeData.textTheme.caption!.color!.withOpacity(0.87))
         .merge(widget.textStyle);

In short, the package required the caption text style to be explicitly declared in the text theme which was causing the null exception. Anyways, thanks for the help!