Klever

During deleting mark (via Marks->Unsafes) the following error is occurred sometimes:

Internal Server Error: /marks/unsafe/
Traceback (most recent call last):
File "/usr/local/lib/python3.4/dist-packages/django/core/handlers/base.py", line 132, in get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/usr/local/lib/python3.4/dist-packages/django/contrib/auth/decorators.py", line 22, in wrapped_view
return view_func(request, *args, **kwargs)
File "/home/vitaly/klever/bridge/marks/views.py", line 305, in mark_list
'tabledata': MarksList(*table_args),
File "/home/vitaly/klever/bridge/marks/tables.py", line 400, in init
self.values = self._get_values()
File "/home/vitaly/klever/bridge/marks/tables.py", line 558, in _get_values
tag.tag.tag for tag in mark.versions.order_by('-version').first().tags.order_by('tag_tag')
AttributeError: 'NoneType' object has no attribute 'tags'

At the same time, the mark is still deleted with this error message.