Help - Recurring 'Error communicating with device'

Hi - help please!

I keep getting the 'Error Communicating with Device' when trying to connect my samsung tablet to calibre. This occurs regardless of which library I have up, and I've made sure I've updated everything fully.

This is what it says:

calibre, version 1.41.0
ERROR: Error: Error communicating with device

'NoneType' object has no attribute 'set'

Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 86, in run
File "site-packages\calibre\gui2\device.py", line 490, in _books
File "site-packages\calibre\devices\smart_device_app\driver.p y", line 50, in _synchronizer
File "site-packages\calibre\devices\smart_device_app\driver.p y", line 1288, in books
AttributeError: 'NoneType' object has no attribute 'set'

Sometimes, though, this error message comes up, and yet there is still a connection between calibre companion and calibre. I can send things to CC, but it doesn't show what is already on my device.

Any ideas?