Did you try to edit a widget in Appearance » Customize » Widgets and once you clicked on the “Apply” button of the widget, the loading image just kept spinning and spinning?

The issue behind that is the Custom Sidebars plugin. It generates a JS error — Uncaught ReferenceError: wpmUi is not defined — that blocks the execution of the widget update.

There are three solutions to this problem:

1. Edit the widgets in Appearance » Widgets instead, where this issue is not present.
2. Deactivate the Custom Sidebars plugin if you are not using it (check if you have any custom sidebars defined in Appearance » Widgets).
3. Use our Custom Sidebars by ProteusThemes plugin and deactivate the other Custom Sidebars plugin.

If you have any custom sidebars already defined in Appearance » Widgets then you can re-create those with our plugin as well. This will allow you to deactivate the Custom Sidebars plugin which causes the edit issues. Our plugin for custom sidebars works OK and will not cause these issues.