When setting a new "Window" menu (as opposed to just modifying the existing
one which did work correctly), we shouldn't show it immediately in the menu
bar if there are no MDI children, this results in wrong UI and assert
failures.
Closes#15663.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75242 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775