save on Event > Info tab never completes, spinning Civi log forever

Saving an Event on the Info tab was spinning indefinitely. The web server logs showed this message:

PHP message: PHP Fatal error:  Uncaught Error: Non-static method CRM_CivirulesActions_Participant_AddToZoom::getJoinUrl() cannot be called statically in /…/ncn-civi-zoom/ncn_civi_zoom.php:310

So, adjusting the getJoinUrl() method to match. MR to follow.