Unset the debug report preview dialog as critical window when it's hidden.

Otherwise no events can be dispatched to any other window after
wxDebugReportPreviewStd::Show() is called, resulting in apparent hanging of
the application as can be seen in debugrpt sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64379 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775