错误详细信息：

System.NullReferenceException

  HResult=0x80004003

  Message=未将对象引用设置到对象的实例。

  Source=mscorlib

  StackTrace:

   在 System.Threading.Tasks.SynchronizationContextAwaitTaskContinuation.PostAction(Object state)

   在 System.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, Object state, Task& currentTask)

--- 上一位置中堆栈跟踪的末尾 ---

   在 System.Threading.Tasks.AwaitTaskContinuation.<>c.<ThrowAsyncIfNecessary>b__18_0(Object s)

   在 System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)

   在 System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   在 System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()

   在 System.Threading.ThreadPoolWorkQueue.Dispatch()

   在 System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

  此异常最初是在此调用堆栈中引发的: 

    [外部代码]