wxApp::OnInit and no top level window

[Chris Mellon]

No matter what the value of SetExitOnFrameDelete, if there's not a top
level window in existence at the time you call MainLoop, your event
loop (and thus the app) will exit immediately, even if you override
OnInit and return True.

This is contrary to the C++ behavior and I can't find any way to start
the main loop without a window existing (although setting
ExitOnFrameDelete to False and destroying the window immediately after
the main loop starts does work).