test_SfpStateUpdateTask_task_run_stop failed

[k-v1]

When I build master branch of sonic-buildimage (sonic-broadcom.bin) sometimes I get this error.

=================================== FAILURES ===================================
____________ TestXcvrdScript.test_SfpStateUpdateTask_task_run_stop _____________

self = <tests.test_xcvrd.TestXcvrdScript object at 0x7f11132cb0d0>

    @patch('xcvrd.xcvrd_utilities.port_mapping.subscribe_port_config_change', MagicMock(return_value=(None, None)))
    def test_SfpStateUpdateTask_task_run_stop(self):
        port_mapping = PortMapping()
        retry_eeprom_set = set()
        stop_event = threading.Event()
        sfp_error_event = threading.Event()
        task = SfpStateUpdateTask(DEFAULT_NAMESPACE, port_mapping, retry_eeprom_set, stop_event, sfp_error_event)
        task.start()
        assert wait_until(5, 1, task.is_alive)
        task.raise_exception()
>       task.join()

tests/test_xcvrd.py:1041: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
xcvrd/xcvrd.py:2177: in join
    raise self.exc
xcvrd/xcvrd.py:2152: in run
    self.task_worker(self.task_stopping_event, self.sfp_error_event)
xcvrd/xcvrd.py:1965: in task_worker
    port_mapping.handle_port_config_change(sel, asic_context, stopping_event, self.port_mapping, helper_logger, self.on_port_config_change)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

sel = None, asic_context = None
stop_event = <threading.Event object at 0x7f1113ccbc40>
port_mapping = <xcvrd.xcvrd_utilities.port_mapping.PortMapping object at 0x7f1113ccbcd0>
logger = <sonic_py_common.logger.Logger object at 0x7f1115fcf970>
port_change_event_handler = <bound method SfpStateUpdateTask.on_port_config_change of <SfpStateUpdateTask(SfpStateUpdateTask, stopped 139711322162944)>>

    def handle_port_config_change(sel, asic_context, stop_event, port_mapping, logger, port_change_event_handler):
        """Select CONFIG_DB PORT table changes, once there is a port configuration add/remove, notify observers
        """
        if not stop_event.is_set():
>           (state, _) = sel.select(SELECT_TIMEOUT_MSECS)
E           AttributeError: 'NoneType' object has no attribute 'select'

xcvrd/xcvrd_utilities/port_mapping.py:218: AttributeError