In trying to debug JENKINS-22157, I found that the GhprbRootAction handler attempts some sort of indirection on the repositories through the getRepos() methods. I suspect there may be failures relating to resolving these repositories, but there are a number of indirection steps with no logging and silent negative returns.

I suggest adding debug logging:

• at line 76, when trying to turn "owner/reponame" into a repository object by browsing through all the jobs on the system, and saying whether we found one
• at line 104, if the trigger repository is null, since it's always a misconfiguration to have the PRB trigger active with no repository
• at lines 56 and 62, possibly at a higher level, if we the returned sets are empty (we are trying to process a hook event but can't find any matching repositories)