NullReferenceException: 未将对象引用设置到对象的实例。
StackTrace:
   在 System.Threading.ThreadPoolWorkQueue.WorkStealingQueue.TrySteal(IThreadPoolWorkItem& obj, Boolean& missedSteal, Int32 millisecondsTimeout)
   在 System.Threading.ThreadPoolWorkQueue.Dequeue(ThreadPoolWorkQueueThreadLocals tl, IThreadPoolWorkItem& callback, Boolean& missedSteal)
   在 System.Threading.ThreadPoolWorkQueue.Dispatch()