Add unit tests for ItemTypeToolStripMenuItem

[Syareel-Sukeri]

Related #10773

Proposed changes

• Add unit test ItemTypeToolStripMenuItemTests.cs for public properties and method of the ItemTypeToolStripMenuItem.
• Enable nullability in ItemTypeToolStripMenuItem.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.47484%. Comparing base (66894e2) to head (f0e3e7f).
Report is 60 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12199         +/-   ##
===================================================
+ Coverage   75.34760%   75.47484%   +0.12724%     
===================================================
  Files           3099        3107          +8     
  Lines         634429      634475         +46     
  Branches       46874       46887         +13     
===================================================
+ Hits          478027      478869        +842     
+ Misses        152989      152167        -822     
- Partials        3413        3439         +26     

Flag	Coverage Δ
Debug	75.47484% <100.00000%> (+0.12724%)	⬆️
integration	17.97674% <0.00000%> (+0.04875%)	⬆️
production	48.89545% <100.00000%> (+0.22043%)	⬆️
test	97.02635% <100.00000%> (-0.00057%)	⬇️
unit	45.93133% <100.00000%> (+0.13831%)	⬆️

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[lonitra]

Thank you