QDialog: don't tell native dialog to hide if never shown

If the platform returns false when asked to show a native
dialog, it means that it could not show it. Calling hide
on it later is therefore wrong, and can cause unwanted
side effects.

Change-Id: Icf5bdb5d98f40af6ef70fdbeaff824a535a404b3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/widgets/dialogs/qdialog.cpp

@@ -127,7 +127,7 @@ bool QDialogPrivate::setNativeDialogVisible(bool visible)
             Q_Q(QDialog);
             helperPrepareShow(helper);
             nativeDialogInUse = helper->show(q->windowFlags(), q->windowModality(), parentWindow());
-        } else {
+        } else if (nativeDialogInUse) {
             helper->hide();
         }
     }