`g_dbus_proxy_call_sync` will return NULL and set the recoverable `error` in case of failure. We didn't catch this previously so reading the variant from the return value caused a CRITICAL because a user didn't have the `week-start-day` setting (introduced in 50.alpha) and the returned `GVariant` was NULL. Now, we check whether en error is set after calling `g_dbus_proxy_call_sync` and propagate it to the calling method so it can gracefully handle it. Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/862>