There was a problem. Updated Events Calendar and LearnPress plugin to the latest, and now it is impossible to add, delete or edit widgets. The error itself is supposedly in the Event Calendar file, BUT, if you disable LearnPress, everything works fine. On the contrary also. Screenshot of error:
Someone faced this?
P.S .: On the default theme (TwentySeventeen) everything also, that is it is not a problem in my theme.
Read more here:: Did you have a conflict with Events Calendar and LearnPress?