CheckIfCanBeUsedAsParent() used by wxMessageBox ctor shouldn't dereference
wxTheApp unconditionally, otherwise it's impossible to show a message box
before creating the application object or after destroying it without crashing.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78233 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775