Fix crash when starting Topic Viewer

[azeey]

🦟 Bug fix

Summary

Starting the Topic Viewer in gz-sim causes a crash for me. The crash only seems to happen in one of my machines, so it might not be easy to reproduce.

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 #574 (8d0515a) into gz-gui8 (c01d327) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 8d0515a differs from pull request most recent head c517328. Consider uploading reports for the commit c517328 to get more accurate results

@@             Coverage Diff             @@
##           gz-gui8     #574      +/-   ##
===========================================
+ Coverage    68.10%   68.11%   +0.01%     
===========================================
  Files           38       38              
  Lines         5367     5369       +2     
===========================================
+ Hits          3655     3657       +2     
  Misses        1712     1712              

Files Changed	Coverage Δ
src/plugins/topic_viewer/TopicViewer.cc	80.89% <100.00%> (+0.24%)	⬆️

[caguero]

I think eventually we should revisit this plugin and also consider visualize subscribers.