Did you tried 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 this Custom Sidebars plugin. It generates a JS error: Uncaught ReferenceError: wpmUi is not defined, which blocks the execution of the widget update.

There are 3 solutions to this problem:

1. you can edit the widgets in Appearance » Widgets, 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 is causing the customizer widget edit issues.

Our plugin for custom sidebars is working OK and will not cause the customizer widget edit issues.