devkinoti said over 2 years ago on FullCalendar with StimulusJS :
Hey David,  I'm using v5.8.0.

Found the solution within their git repository closed issues: https://github.com/fullcalendar/fullcalendar/issues/5621
Ideally the error is tied to whether you have a valid DOM object or not.

A simple typo triggers it.

The TypeError: Cannot read property '__k' of undefined is a slightly misleading/unclear error message.