When the until's time part is invalid, the libical claims the time
as valid, but the conversion into the GDateTime fails and returns NULL.
Such events, which claim to have until-date recurrence, but NULL 'until'
cannot be edited, because they cause a crash of the application.
More information can be found downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=2135772
Related to https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/892
Milan Crha