SNEWS alert TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'

https://emfollow.ligo.caltech.edu/sentry/gwcelery/issues/54/

TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'
  File "celery/app/trace.py", line 382, in trace_task
    R = retval = fun(*args, **kwargs)
  File "celery/app/trace.py", line 641, in __protected_call__
    return self.run(*args, **kwargs)
  File "gwcelery/tasks/gcn/external_triggers.py", line 53, in handle
    end = start + event['extra_attributes']['GRB']['trigger_duration']

TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'