Comment 1 for bug 1951225

Looks like a problem when instantiating singleton agent cache

2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event [req-1182c635-4905-4062-a060-6e445fe25d25 - - - - -] Unexpected exception in notify_loop: ValueError: driver cannot be None
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event Traceback (most recent call last):
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/ovsdbapp/event.py", line 148, in notify_loop
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event match.run(event, row, updates)
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event File "/home/zuul/src/opendev.org/openstack/neutron/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovsdb_monitor.py", line 333, in run
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event n_agent.AgentCache().update(ovn_const.OVN_CONTROLLER_AGENT, row,
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event File "/home/zuul/src/opendev.org/openstack/neutron/neutron/common/utils.py", line 941, in __call__
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event self._instance = self._klass(*args, **kwargs)
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event File "/home/zuul/src/opendev.org/openstack/neutron/neutron/plugins/ml2/drivers/ovn/agent/neutron_agent.py", line 203, in __init__
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event raise ValueError(_("driver cannot be None"))
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event ValueError: driver cannot be None
2021-11-15 15:00:09.556 52726 ERROR ovsdbapp.event