Good question. 

Now digging deeper I see that comment on above block is actually lying a bit.

What happens on creating an event to an excluded default notebook:
1. NemoCalendarEvent::save() is called, does NemoCalendarDb::calendar()->addEvent().
2. above does NemoCalendarDb::storage()->save();

In the middle and after them the calendar contains the newly created event. Returns valid event with calendar->event(uid);

3. QFileSystemWatcher in sqlitestorage kicks in. Deletes all notebooks, NemoCalendarEventCache::storageModified() is called which does load().
4. Above skips loading the excluded default notebook.

Result: valid event no longer returned on calendar->event().

Coming back to printing error messages. Interface itself doesn't disallow creating events to an excluded notebook, so some of this could be expected. In that sense I could maybe remove qWarning() below. It would allow matching event later when exclusion is removed.

But maybe this branch should indeed have the warning. If Nemo calendar event has null kcalcore event, it has already lost uniqueId which doesn't sound good.