[2.2] Failure while handling notification to 'event'

I saw the following traceback[1] after doing the following:

(1) Add a pod and letting the pod add machines, which moved to "Commissioning" state.
(2) Delete all the machines that were added.
(3) Go to the pod details and select "Refresh". Wait for the machines in the pod to be discovered.
(4) Select all the machines in "Commissioning" state using the UI, and then delete them.
(5) Deleting the pod itself.

If it matters, in the process I may have closed or refreshed the browser window, since it didn't seem to be responding or updating the UI.

[1]:

http://paste.ubuntu.com/24284847/

2017-03-31 01:08:11 maasserver.listener: [critical] Failure while handling notification to 'event': '44764'

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 434, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/maasserver/websockets/protocol.py", line 421, in onNotify
    self.processNotify, handler, channel, action, obj_id)
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 855, in callInContext
    return func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 223, in wrapper
    result = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 686, in call_within_transaction
    return func_outside_txn(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 504, in retrier
    return func(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/maasserver/websockets/protocol.py", line 428, in processNotify
    return handler.on_listen(channel, action, obj_id)
  File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/event.py", line 112, in on_listen
    obj = self.listen(channel, action, pk)
  File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 526, in listen
    self._meta.pk: pk
  File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 287, in get_object
    raise HandlerDoesNotExistError(pk)
maasserver.websockets.base.HandlerDoesNotExistError: 44764