Fixes #3754. UICatalog throws System.NullReferenceException on MenuBar in Linux.

[BDisp]

Fixes

• Fixes UICatalog throws System.NullReferenceException on MenuBar in Linux. #3754

Proposed Changes/Todos

• IsEnabled method now checks for null MenuItem.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

Nice find and fix. Thanks.