(process:2937): GLib-GIO-ERROR **: 19:33:19.399: Settings schema 'org.gnome.calendar' is not installed
I've been seeing these kinds of errors more frequently lately, running Ubuntu 18.04 LTS.
Can't tell the root cause, but was able to fix the problem by reinstalling the app and desktop schema packages, and then rebuilding the schemas. So, for gnome-calendar:
$ sudo apt install --reinstall gnome-calendar
$ sudo apt install --reinstall gsettings-desktop-schemas
$ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
I also had to run this command when the calendar icon failed to show up in the applications menu:
$ sudo update-icon-caches /usr/share/icons/*