Winforms stack trace dialog buttons are unreliable

[mhsmith]

Steps to reproduce

Open the dialogs example, and click "Stack Trace". The dialog appears, and then 1 second later, this appears in the console:

Task was destroyed but it is pending!
task: <Task pending name='Task-1' coro=<handler_with_cleanup() done, defined at C:\Users\smith\cygwin\git\beeware\toga\core\src\toga\handlers.py:36> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x000002612A605880>()]>>

Click the OK button. Nothing happens.

Click the close button. The dialog closes.

This only happens the first time you open the dialog in a process. In all subsequent dialogs, the OK button closes the dialog but the close button does not.

Expected behavior

Based on the code, it looks like the intention is that the OK button closes the dialog but the close button does not.

Environment

• Operating System: Windows 10
• Python version: 3.8
• Software versions:
  • Toga: 4a27e6e