Python loopback throws spurious errors on shutdown

These errors don't appear to actually cause any problems, but they are misleading. Looks like we're trying to stop subprocesses even when there are no subprocesses.
 
ERROR:grpc._server:Exception calling application: u'1-1'
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/grpc/_server.py", line 434, in _call_behavior
    response_or_iterator = behavior(argument, context)
  File "/usr/local/lib/python2.7/dist-packages/apache_beam-2.17.0.dev0-py2.7.egg/apache_beam/runners/worker/worker_pool_main.py", line 126, in StopWorker
    worker_process = self._worker_processes.pop(stop_worker_request.worker_id)
KeyError: u'1-1'