[release/10.0] Fix Control.Dispose crash when dependent UserControl fails to load due to missing assembly

[github-actions]

Backport of #13840 to release/10.0
Fixes #14393

Customer Impact

This change ensures that Control.Dispose() can safely execute even when the control was never fully constructed.

Fix

Added a defensive null check around ContextMenuStrip access in Control.Dispose()
Replaced direct property access with Properties?.TryGetValue(...) to ensure safe event unsubscription

Testing

Manual with the customer repro case.

Risk

Low - This fix only adds a defensive null check and will not affect existing logic.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.11109%. Comparing base (ee90158) to head (60b202d).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           release/10.0      #14398         +/-   ##
======================================================
+ Coverage      77.10644%   77.11109%   +0.00464%     
======================================================
  Files              3273        3273                 
  Lines            645033      645033                 
  Branches          47701       47702          +1     
======================================================
+ Hits             497362      497392         +30     
+ Misses           143989      143964         -25     
+ Partials           3682        3677          -5     

Flag	Coverage Δ
Debug	77.11109% <0.00000%> (+0.00464%)	⬆️
integration	18.98850% <0.00000%> (+0.00870%)	⬆️
production	51.93587% <0.00000%> (+0.01042%)	⬆️
test	97.40998% <ø> (ø)
unit	49.35089% <0.00000%> (+0.00764%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.