Cannot delete a mount point

Hi,

After creating a jail with the UI, I created a mount point. All good.

Then I wanted to delete that mount point which failed giving the following error:

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 166, in call_method
result = await self.middleware.call_method(self, message)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1097, in call_method
return await self._call(message['method'], serviceobj, methodobj, params, app=app, io_thread=False)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1042, in _call
return await self._call_worker(name, *args)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1062, in _call_worker
return await self.run_in_proc(main_worker, name, args, job)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 996, in run_in_proc
return await self.run_in_executor(self.__procpool, method, *args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 972, in run_in_executor
return await loop.run_in_executor(pool, functools.partial(method, *args, **kwargs))
TypeError: object of type 'NoneType' has no len()