sfDependentSelectPlugin: Cannot set property 'onchange' of null

From README:

        // always the selects order has to be according to the dependency.
        // ich means: country > state > city
        $this->widgetSchema->moveField('city_id', 'after', 'state_id');

If you not set the order, you will get a Uncaught TypeError: Cannot set property 'onchange' of null error.