ListDetailsView.ListPaneWidth Null check fix

[LalithaNadimpalli]

List Details View NRE check

Fixes #4357

PR Type

What kind of change does this PR introduce?

< Bugfix >

< Sample app changes >

What is the current behavior?

<_TwoPaneView passed without checking for null>

What is the new behavior?

PR Checklist

Please check if your PR fulfills the following requirements:

• 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
• If control, added to Visual Studio Design project
• Sample in sample app has been added / updated (for bug fixes / features)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes

[ghost]

Thanks LalithaNadimpalli 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 🙌

[Arlodotexe]

Repro'd the issue, swapped the nuget package for this branch locally, and the issue was fixed.

Looks good!

[pszyjaciel]

ListPaneWidth crashes with NullReferenceException. Issue still exists in WinUI3 Microsoft.WindowsAppSDK 1.5.240311000, Microsoft.UI.Xaml 2.8.6