Calendar not show after update

Remove the “defer” in the line where the fullcalendar core component is being loaded - for example:

<script src="https://unpkg.com/@fullcalendar/core@4.4.2/main.min.js" defer></script>

…so that it looks like:

<script src="https://unpkg.com/@fullcalendar/core@4.4.2/main.min.js"></script>

Then it should work.