API docs for Bus.add_watch()/add_watch_full() show the method name as
add_watch_full(). However, as this method shadows
add_watch(), gjs exposes the function as
add_watch(), but it takes the same arguments as
add_watch_full(), ie the first argument should be a numerical priority. Trying to call a method
add_watch(...) causes an error, because gjs can not find that symbol on Bus' prototype.