fix: handle Null Dereference in UnitsConverter

[vil02]

Continuation of #5456.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.17%. Comparing base (3fe1de0) to head (8cf21f7).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5460      +/-   ##
============================================
- Coverage     52.19%   52.17%   -0.02%     
- Complexity     3244     3245       +1     
============================================
  Files           524      524              
  Lines         15158    15158              
  Branches       2881     2881              
============================================
- Hits           7912     7909       -3     
- Misses         6922     6924       +2     
- Partials        324      325       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.