Tuned throws an exception when adding a new logical volume or hotplugging a disk

Issue

When a new logical volume is created or a new disk is hotplugged , tuned throws an exception message like the following one:

2019-07-23 15:05:50,216 INFO     tuned.plugins.hotplug: device 'dm-20' added
2019-07-23 15:05:50,217 INFO     tuned.plugins.hotplug: instance disk: adding new device dm-20
2019-07-23 15:05:50,217 ERROR    tuned.hardware.inventory: Exception occured in event handler of '<tuned.plugins.plugin_disk.DiskPlugin object at 0x7ff738a5df10>'.
2019-07-23 15:05:50,217 ERROR    tuned.hardware.inventory: 
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/tuned/hardware/inventory.py", line 58, in _handle_udev_event
    callback(event, device)
  File "/usr/lib/python2.7/site-packages/tuned/plugins/plugin_disk.py", line 54, in _hardware_events_callback
    super(DiskPlugin, self)._hardware_events_callback(event, device)
  File "/usr/lib/python2.7/site-packages/tuned/plugins/hotplug.py", line 29, in _hardware_events_callback
    self._add_device(device)
  File "/usr/lib/python2.7/site-packages/tuned/plugins/hotplug.py", line 45, in _add_device
    self._added_device_apply_tuning(instance, device_name)
  File "/usr/lib/python2.7/site-packages/tuned/plugins/plugin_disk.py", line 58, in _added_device_apply_tuning
    instance._load_monitor.add_device(device_name)
  File "/usr/lib/python2.7/site-packages/tuned/monitors/base.py", line 102, in add_device
    assert isinstance(device, str)
AssertionError