MimeType text/calendar is missing from the desktop file
At calendar v3.28.2 used in 18.04 Ubuntu LTS, the gnome-calendar desktop file had a "MimeType=text/calendar;" entry.
Looking at v3.31.90 used in Ubuntu 19.04 that is now missing which I believe means you cannot set GNOME-Calendar to be the default calendar in GNOME-Control-Center - default applications - calendar.
Can this be readded?