This fixes a problem with deleting an unmounted package instance. If you added and mounted, say, a subsite instance, unmounted and then attempted to delete it through sitemap->Manage unmounted applications, you would get an error. It seems site_node::get_node_id_from_object_id doesn't even fail for an unmounted package, returning the empty string in this case, and the catch test doesn't trip. Now we explicitly test for the node_id being set to a sensible value.