Add check to makesure `self.duration` exists before using it.
Add check to makesure `self.duration` exists before using it.

ic-labs/django-icekit,ic-labs/icekit-events,ic-labs/icekit-events,ic-labs/django-icekit,ic-labs/django-icekit,ic-labs/django-icekit,ic-labs/icekit-events