NPE during closing down of the toethread

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Sep 21, 2016 11:16:43 AM org.archive.crawler.framework.ToeThread run
SEVERE: Fatal exception in ToeThread #12: http://fh6x.com/301/taohua.png?WebShieldDRSessionVerify=E4n0zdtYjo6ID4wtek7M (in thread 'ToeThread #12: http://fh6x.com/301/taohua.png?WebShieldDRSessionVerify=E4n0zdtYjo6ID4wtek7M')
java.lang.NullPointerException
	at org.archive.crawler.frontier.BdbMultipleWorkQueues.delete(BdbMultipleWorkQueues.java:484)
	at org.archive.crawler.frontier.BdbWorkQueue.deleteItem(BdbWorkQueue.java:88)
	at org.archive.crawler.frontier.WorkQueue.dequeue(WorkQueue.java:195)
	at org.archive.crawler.frontier.WorkQueueFrontier.processFinish(WorkQueueFrontier.java:948)
	at org.archive.crawler.frontier.AbstractFrontier.finished(AbstractFrontier.java:574)
	at org.archive.crawler.framework.ToeThread.run(ToeThread.java:187)