App crashes when the last item is deleted

[lchamp]

Issue description

The watchOS app crashes when the last event is deleted from the list.

Steps to reproduce the issue

1. Add one (or more) event to the list
2. Remove them one by one using the swipe-to-delete gesture
3. Note that when the last one is deleted the behavior is a crash

What's the expected result?

• Application should not crash
• We should see the (empty) list and the Create Event button.

What's the actual result?

• Application crash, returns to the watch apps list

*** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndexedSubscript:]: index 0 beyond bounds for empty array'