Fix crash after 24926b2e23.

The matched element may not have a parent. I guess the old version of
setExpanded too care of that. Now we have to do so.

src/frontends/qt/GuiPrefs.cpp

@@ -3266,6 +3266,7 @@ void PrefShortcuts::on_searchLE_textEdited()
 	for (int i = 0; i < matched.size(); ++i)
 		if (!isAlwaysHidden(*matched[i])) {
 			matched[i]->setHidden(false);
+			if (matched[i]->parent())
 				matched[i]->parent()->setExpanded(true);
 		}
 }