'How do I get an event using fullcalendar getEventSourceById?

The function as mentioned in the docs doesn't seem to be working:

Events are added with ids 1 and 2.
I execute calendar.getEventSourceById(1)
I expect to get the first event

To reproduce see here: https://codesandbox.io/s/full-calendar-demo-forked-iihhvn

fullcalendar fullcalendar-5

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