This counterintuitive behaviour could happen when a dialog of a frame
returning false from its ShouldPreventAppExit() was closed: if there were no
other TLWs in the application, then the parent frame could be closed as well,
even if this clearly shouldn't happen.
See #15880.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75633 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775