Add pointer check in Application::RemovePlugin

[nkoenig]

Signed-off-by: Nate Koenig [email protected]

🦟 Bug fix

Summary

Adds a pointer check in the Application::RemovePlugin function

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #501 (16c5f74) into ign-gui3 (b09d084) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 16c5f74 differs from pull request most recent head 42b9b20. Consider uploading reports for the commit 42b9b20 to get more accurate results

@@            Coverage Diff            @@
##           ign-gui3     #501   +/-   ##
=========================================
  Coverage     73.82%   73.82%           
=========================================
  Files            30       30           
  Lines          3270     3270           
=========================================
  Hits           2414     2414           
  Misses          856      856           

Impacted Files	Coverage Δ
src/Application.cc	87.50% <100.00%> (ø)

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