The most likely cause of the crash is that a timer fired, after it was closed.

MSDN documents such a possibility, in the documentation for
CloseThreadpoolTimer() function, and recommends disabling the timer before
calling WaitForThreadpoolTimerCallbacks()/CloseThreadpoolTimer().

The fix follows this recommendation.

Note, that 5.5-10.1 disabled the timer before close, but this code
was lost in threadpool refactoring in 10.2