After deleting a cell in a notebook I always get an error message. (After manual refresh its fine)
Browser: Safari
Apologies. I think one of our recent fixes (for hiding empty cells) caused this regression. I will investigate shortly but I won’t be able to fix it for a couple hours.
We pushed a fix! 
Thank you for the report, and please let us know if you encounter any further quirks, crashes, bugs, etc.