Use of undefined constant rs_event_widgets – assumed ‘rs_event_widgets’

when changing to PHP 7.2.25, I got this warning
Warning: Use of undefined constant rs_event_widgets – assumed ‘rs_event_widgets’ (this will throw an Error in a future version of PHP) in /var/www/<myurl>/wp-content/plugins/rs-event-multiday/rs-event-multiday.php on line 1314

I solved the problem, by thanging this in /rs-event-multiday.php line 1314

/* register widget when loading the WP core */
add_action(‘widgets_init’, rs_event_widgets);

/* register widget when loading the WP core */
add_action(‘widgets_init’, rs_event_widgets’);