This NPE may happen in the case of race condition.

1) AbstractProject#getCauseOfBlockage() checks for non-concurrent running build in order to generate a cause of blockage
2) If the getCauseOfBlockage() happens in the context not synced with queue, after isLogUpdated() && !isConcurrentBuild() there may be a context switch.
3) Build may complete and get deleted before we actually call getLastBuild() in the code
4) If it happens, BlockedBecauseOfBuildInProgress may get null in the constructor
5) Then, BlockedBecauseOfBuildInProgress#getDescription() will start throwing NPEs