fix: Fix color picker button binding failure issue

[HavenDV]

Fixes

Fixes #4721

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

The current SelectedValue binding throws a Binding Failure error because it tries to return a null Color property (due to BindingMode=TwoWay) that has a value type

What is the new behavior?

Added a helper converter that will ignore null values in ConvertBack, replacing them with DependencyProperty.UnsetValue

PR Checklist

Please check if your PR fulfills the following requirements:

• [x Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
  • If control, added to Visual Studio Design project
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

[ghost]

Thanks HavenDV for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[michael-hawker]

@robloo mind having a quick look if you have a few minutes?

[robloo]

I'll look at the last few issues and this together. Please give me this weekend to do it though.

[robloo]

I confirmed the issue and this fix looks fine to me. It just never allows setting null to the non-nullable ColorPicker.Color property which would otherwise occur when no color is selected in the palette.

It is unfortunate another converter is needed for this but I can't think of a better way.

[robloo]

Note that I made a more general-purpose converter in Avalonia to fix this. I'm not asking for any changes here but just FYI:

https://github.com/AvaloniaUI/Avalonia/pull/8829/files

[niels9001]

Tested in the sample app by setting the SelectedColor to {x:Null}. It was throwing binding errors in the current build, this PR resolved that 😊. Great work @HavenDV - LGTM!