Allow wxPreferencesEditor::Dismiss() to work when using modal dialogs too.

The modal dialog case is not really different, the dialog may still need to be
dismissed if the associated object doesn't exist any longer.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74010 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775