NilObjectException for Thread

My application have two window and I have thread1 in one of window. When I call showModule for the window that have thread, is working fine. However, If I try to close the application, the NilObjectException occurs for thread1. I think this error is calling destructor for thread1 when the application is closing, but I do not understanding why thread1 is Nil during the application is closing. Can anyone tell me what happen or how this happen to the thread1 when an application is closing?

If Thread1 is an instance that you dragged onto the window, it ceases to exist when the window is closed.