Write a unit test that handles both cases to create the matrix aggregator.
Are you still intending to work on this?
I would like to work on this issue
Well, then go ahead. The class is quite simple. The complex thing is that you need to create a stub with Mockito to get the if branch correctly tested.
Sorry for being late. Made the PR here https://github.com/jenkinsci/warnings-ng-plugin/pull/662