Do prevent the app from exiting when the help frame is shown in the special
case of the help viewer application where the default behaviour of exiting the
application when only the help window remains open is inappropriate and
results in unexpected program exit if we open and then close any dialog, e.g.
from the help frame toolbar.
Even though the previous commit actually already fixed the problem in the
current code, still apply this fix as well, just in case we can (or could, in
the future) should any dialogs with NULL parent in this program.
Closes#15880.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75634 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
9084dd3fb0