The bug is also reproduced in our environment.

2018-12-18 18:43:33.807 101283 DEBUG ovsdbapp.backend.ovs_idl.transaction [-] Transaction caused no change do_commit /usr/lib/python2.7/site-packages/ovsdbapp/backend/ovs_idl/transaction.py:121
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp [req-f4683b4e-3437-4d13-b0ea-63b7a065ef5d - - - - -] Agent main thread died of an exception: KeyError: 1
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp Traceback (most recent call last):
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/openvswitch/agent/openflow/native/ovs_ryuapp.py", line 40, in agent_main_wrapper
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp     ovs_agent.main(bridge_classes)
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py", line 2272, in main
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp     agent = OVSNeutronAgent(bridge_classes, ext_mgr, cfg.CONF)
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py", line 241, in __init__
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp     self._restore_local_vlan_map()
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py", line 358, in _restore_local_vlan_map
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp     self.available_local_vlans.remove(local_vlan)
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp KeyError: 1
2018-12-18 18:43:33.807 101283 ERROR neutron.plugins.ml2.drivers.openvswitch.agent.openflow.native.ovs_ryuapp
2018-12-18 18:43:33.810 101283 CRITICAL neutron [-] Unhandled error: KeyError: 1
2018-12-18 18:43:33.810 101283 ERROR neutron Traceback (most recent call last):
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/bin/neutron-openvswitch-agent", line 10, in <module>
2018-12-18 18:43:33.810 101283 ERROR neutron     sys.exit(main())
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/neutron/cmd/eventlet/plugins/ovs_neutron_agent.py", line 20, in main
2018-12-18 18:43:33.810 101283 ERROR neutron     agent_main.main()
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/openvswitch/agent/main.py", line 47, in main
2018-12-18 18:43:33.810 101283 ERROR neutron     mod.main()
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/openvswitch/agent/openflow/native/main.py", line 35, in main
2018-12-18 18:43:33.810 101283 ERROR neutron     'neutron.plugins.ml2.drivers.openvswitch.agent.'
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/ryu/base/app_manager.py", line 375, in run_apps
2018-12-18 18:43:33.810 101283 ERROR neutron     hub.joinall(services)
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/ryu/lib/hub.py", line 103, in joinall
2018-12-18 18:43:33.810 101283 ERROR neutron     t.wait()
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/eventlet/greenthread.py", line 175, in wait
2018-12-18 18:43:33.810 101283 ERROR neutron     return self._exit_event.wait()
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/eventlet/event.py", line 125, in wait
2018-12-18 18:43:33.810 101283 ERROR neutron     current.throw(*self._exc)
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/eventlet/greenthread.py", line 214, in main
2018-12-18 18:43:33.810 101283 ERROR neutron     result = function(*args, **kwargs)
2018-12-18 18:43:33.810 101283 ERROR neutron   File "/usr/lib/python2.7/site-packages/ryu/lib/hub.py", line 65, in _launch
2018-12-18 18:43:33.810 101283 ERROR neutron     raise e