Exception caught with scheduler

When trying to add new skills to a stage we experience the error below after a certain number of tries. I restarted our JIRA instance and the problem went away for a bit, but my coworker started working on evaluating the plugin again but the exception started occurring for him again.

2015-08-26 15:46:01,455 pool-14-thread-1 ERROR xxxxx 946x151x1 1vv4m18 10.97.xx.xx /rest/roadmap/1.0/scheduling/4.json [roadmap.common.scheduling.Calculation] Caught exception in scheduling algorithm.
2015-08-26 15:46:01,457 pool-14-thread-1 ERROR xxxxx 946x151x1 1vv4m18 10.97.xx.xx /rest/roadmap/1.0/scheduling/4.json [roadmap.common.scheduling.Calculation]
java.lang.NullPointerException
at com.radiantminds.roadmap.common.scheduling.trafo.backlog.NoAssignmentPoolViolationDetector.handleInvalidAssignment(NoAssignmentPoolViolationDetector.java:28)
at com.radiantminds.roadmap.common.scheduling.trafo.backlog.ViolationDetector.tryFindViolation(ViolationDetector.java:48)
at com.radiantminds.roadmap.common.scheduling.trafo.backlog.AssignmentTransformer.getWorkViolations(AssignmentTransformer.java:56)
at com.radiantminds.roadmap.common.scheduling.trafo.backlog.AssignmentTransformer.tryTransformAssignment(AssignmentTransformer.java:47)
at com.radiantminds.roadmap.common.scheduling.trafo.backlog.WorkItemTransformer.transform(WorkItemTransformer.java:58)
at com.radiantminds.roadmap.common.scheduling.trafo.backlog.BacklogTransformer.transformBacklog(BacklogTransformer.java:94)
at com.radiantminds.roadmap.common.scheduling.trafo.RoadmapProblemTransformer.createProblemDataSource(RoadmapProblemTransformer.java:81)
at com.radiantminds.roadmap.common.scheduling.LocalCalculationProcess.execute(LocalCalculationProcess.java:83)
at com.radiantminds.roadmap.common.scheduling.Calculation$2.run(Calculation.java:135)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)