Our Jenkins froze after below exceptions which occurred about 200 times per an hour.

After restarting Jenkins, these exceptions haven't occurred.

What is the root cause of these errors?

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18-May-2017 10:11:14.093 SEVERE jenkins.util.Timer [#6] hudson.triggers.SafeTimerTask.run Timer task hudson.slaves.NodeProvisioner$NodeProvisionerInvoker@52943bdb failed
java.lang.NullPointerException

18-May-2017 10:11:20.547 SEVERE jenkins.util.Timer [#2] hudson.triggers.SafeTimerTask.run Timer task hudson.model.Queue$MaintainTask@3252b695 failed
java.lang.NullPointerException

18-May-2017 10:18:37.229 SEVERE jenkins.util.Timer [#4] hudson.triggers.SafeTimerTask.run Timer task hudson.model.LoadStatistics$LoadStatisticsUpdater@63aca18d failed
java.lang.NullPointerException
at hudson.model.ParametersAction.getAssignedLabel(ParametersAction.java:185)
at hudson.model.Queue$Item.getAssignedLabelFor(Queue.java:2053)
at hudson.model.Label$1.matches(Label.java:120)
at hudson.model.LoadStatistics.computeSnapshot(LoadStatistics.java:363)
at hudson.model.LoadStatistics$LoadStatisticsUpdater.doRun(LoadStatistics.java:397)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:50)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)