New class called PlotBuildAction was added when compatibility with Pipeline plugin was implemented in https://github.com/jenkinsci/plot-plugin/pull/32.
Seems like this class current implementation is not thread safe and user reported 1st time issue with ConcurrentModificationException in PR comment. Exception was not reproducible to me even though I used provided by reporter Jenkins & Pipeline configurations.
There was an attempt to blindly fix the issue in commit, but it didn't work out.
Second report about the same exception appeared recently.
- We need to reproduce ConcurrentModificationException while accessing plots array in PlotBuildAction and fix it.
- It's good to have a test for the fix