This used to work, at least in wxMSW, but stopped working after the
(perfectly valid, on their own) changes of 2508efdd6e (Initialize
wxMemoryDC with a default font, 2019-08-13), as this resulted in calling
wxApp::GetTopWindow() that can only be called from the GUI code.
Fix this by adding wxApp::GetGUIInstance() and using it in
GetMainTopWindow(), so that we only call GetTopWindow() if we actually
have a GUI wxApp object on which to call it.
Implement this in terms of a new virtual IsGUI() which seems slightly
better than, although roughly equivalent to, using wxDynamicCast().
Closes https://github.com/wxWidgets/wxWidgets/pull/2617Closes#19343.
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