Event.from_event causes IndexError for events which do not have a category

[RyanBalfanz]

IndexError: list index out of range

Stacktrace (most recent call last):

  File "django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/csrf.py", line 77, in wrapped_view
    return view_func(*args, **kwargs)
  File "sendgrid/views.py", line 134, in listener
    response = handle_single_event_request(request)
  File "sendgrid/views.py", line 48, in handle_single_event_request
    emailMessage = EmailMessage.from_event(eventData)
  File "sendgrid/models.py", line 216, in from_event
    "category": categories[0],