TypeError: _calc_indents(): Argument #4 ($item) must be of type Drupal\calendar\CalendarEvent, array given

Yes, this is my mistake, I blindly trusted this typehint:
https://git.drupalcode.org/project/calendar/-/merge_requests/60/diffs?co...

And that could very well be true that it sometimes is that value, if it's not the ->getStartDate() 3 lines lower will fail.

Thanks @ramil g, I will merge this in right away.