I have got the same notice for my site in WordPress 5.0 on PHP 7.3. Until PHP 7.2 everything works fine.
Hi, Marcus, thanks a lot for your very fast help. Now, your plugin is perfect again. Five stars!
Hi, after updating to version 5.7, in my configuration (multisite with common tables! under PHP 7.1) new events are not stored correctly. In the table wp_em_events, the field blog_id shows «null».