Can't edit Main menu block in sidebar by clicking on "gear"
It is caused by legacy functionality in old versions of jQuery no longer being supported in more modern versions. In this particular case I think it has to do with browser version detection, which has fallen out of favour for feature detection. 

Add the jquery-migrate script to your theme's javascript folder and load it via your theme's info file.

If you continue to have issues you might need to make sure this file loads directly after the jQuery main file.