Python Precommit fail: 'BeamFnExternalWorkerPoolServicer' has no attribute '_worker_processes'

Logs: https://builds.apache.org/job/beam_PreCommit_Python_Commit/8246/console

10:14:09 ----------------------------------------------------------------------
10:14:09 XML: /home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py2/build/srcs/sdks/python/nosetests.xml
10:14:09 ----------------------------------------------------------------------
10:14:09 Ran 2594 tests in 629.438s
10:14:09
10:14:09 OK (SKIP=520)
10:14:09 Error in atexit._run_exitfuncs:
10:14:09 Traceback (most recent call last):
10:14:09 File "/usr/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
10:14:09 func(*targs, **kargs)
10:14:09 File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py2/build/srcs/sdks/python/apache_beam/runners/worker/worker_pool_main.py", line 72, in kill_worker_processes
10:14:09 for worker_process in cls._worker_processes.values():
10:14:09 AttributeError: type object 'BeamFnExternalWorkerPoolServicer' has no attribute '_worker_processes'
10:14:09 Error in sys.exitfunc:
10:14:09 Traceback (most recent call last):
10:14:09 File "/usr/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
10:14:09 func(*targs, **kargs)
10:14:09 File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py2/build/srcs/sdks/python/apache_beam/runners/worker/worker_pool_main.py", line 72, in kill_worker_processes
10:14:09 for worker_process in cls._worker_processes.values():
10:14:09 AttributeError: type object 'BeamFnExternalWorkerPoolServicer' has no attribute '_worker_processes'
10:14:10 py27-cython run-test-post: commands[0] | /home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py2/build/srcs/sdks/python/scripts/run_tox_cleanup.sh
10:14:10 ___________________________________ summary ____________________________________