prevent ExitDialog from being destroyed twice

[seanbudd]

Link to issue number:

None

Summary of the issue:

the following error happens when exiting NVDA via NVDA+q exit dialog shows, then enter:

DEBUG - core._closeAllWindows (18:25:34.959) - MainThread (8636):
destroying main frame during exit process
ERROR - unhandled exception (18:25:34.959) - MainThread (8636):
Traceback (most recent call last):
  File "C:\work\repo\nvda\7\.venv\lib\site-packages\wx\core.py", line 3407, in <lambda>
    lambda event: event.callable(*event.args, **event.kw) )
RuntimeError: wrapped C/C++ object of type ExitDialog has been deleted

Description of how this pull request fixes the issue:

Prevents ExitDialog from being destroyed twice

Testing strategy:

Manual testing

Known issues with pull request:

None

Change log entries:

None, regression

Code Review Checklist:

• Pull Request description is up to date.
• Unit tests.
• System (end to end) tests.
• Manual tests.
• User Documentation.
• Change log entry.
• Context sensitive help for GUI changes.

[AppVeyorBot]

• Build (for testing PR)

See test results for failed build of commit fd0958ecda

[feerrenrut]

Looks good.