Hello,
This error is most likely caused by a delayed reply to ajax request.
What happens is you're on screen A, an ajax request is made to the server, but before the answer arrives, you switch to screen B. The reply to ajax request is a javascript that tries to operate on an element on the page that no longer exists.
We try to add failsafe mechanism to such requests, but it's possible that in few places we missed them.
I found the one in translations and fixed it. It's not a critical bug fix, but if you prefer I can provide a fix for it (the error is not causing any other issues than the popup you see).
Can you please provide more details on when that happens? In particular - what screen are you leaving when this happens?
Kind regards,
Arek
