Fix potential NPEs in BaseMenuManager

[bilal-alsharifi]

Fixes #1215

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

Try setting the menu to null and make sure the menu cells are getting cleared correctly without crashing the app:

sdlManager.getScreenManager().setMenu(null);

Summary

This PR fixes some potential NPEs in MenuManager when setting the menu to null

CLA

• I have signed the CLA

[codecov-io]

Codecov Report

Merging #1216 into hotfix/4.10.1 will increase coverage by <.01%.
The diff coverage is 66.66%.

@@                Coverage Diff                 @@
##             hotfix/4.10.1   #1216      +/-   ##
==================================================
+ Coverage             47.1%   47.1%   +<.01%     
- Complexity            4327    4328       +1     
==================================================
  Files                  481     481              
  Lines                23475   23479       +4     
  Branches              2683    2685       +2     
==================================================
+ Hits                 11057   11059       +2     
  Misses               11750   11750              
- Partials               668     670       +2

Impacted Files	Coverage Δ	Complexity Δ
...vicelink/managers/screen/menu/BaseMenuManager.java	45.12% <66.66%> (+0.03%)	91 <0> (+1)	⬆️

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