tdf#105017 Crash when click a "New Theme..." in Gallery on detached SideBar

Change-Id: I6a3c44a0a08cf036776976d40c985889698c08ac
Reviewed-on: https://gerrit.libreoffice.org/33041Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
(cherry picked from commit a076a062)
Reviewed-on: https://gerrit.libreoffice.org/33056Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>

vcl/source/window/dialog.cxx

@@ -1039,7 +1039,7 @@ void Dialog::SetModalInputMode( bool bModal )
         // Disable the prev Modal Dialog, because our dialog must close at first,
         // before the other dialog can be closed (because the other dialog
         // is on stack since our dialog returns)
-        if ( mpPrevExecuteDlg && !mpPrevExecuteDlg->IsWindowOrChild( this, true ) )
+        if ( mpPrevExecuteDlg && !mpPrevExecuteDlg->isDisposed() && !mpPrevExecuteDlg->IsWindowOrChild( this, true ) )
             mpPrevExecuteDlg->EnableInput( false, this );
 
         // determine next overlap dialog parent