Add null check for ListDetailsView.OnListPaneWidthChanged

[nodew]

Fixes

When I try to use the ListPaneWidth of ListDetailsView with WinUI3/WindowsAppSDK, I got the same error described in #4357
It seems that the fix #4738 only fixed the issue in UWP and missed WinUI3

PR Type

What kind of change does this PR introduce?

Bugfix

Refactoring

What is the current behavior?

_TwoPaneView passed without checking for null

What is the new behavior?

It will check if _twoPaneView is null, and set value to _twoPaneView.Pane1Length only when it's not null.

PR Checklist

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• 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
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  • 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
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• 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 nodew 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 🙌

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[net-foundation-cla]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

❌ nodew sign now
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[nodew]

@michael-hawker @azchohfi Could you please help review the changes?

[michael-hawker]

@nodew see message here: #4800 (reply in thread)

rel/winui/7.1.2 is a release branch, so we won't directly update it with a fix like this. The winui branch never got updated with the fixes from the main branch yet due to the issues with getting it to build again in #4752.

Our plan is to fix everything forward for 8.0 as we bring things over to our maintainable infrastructure.

[nodew]

@michael-hawker Good to know that, thanks for the information.