It surely did not escape your notice that WordPress updated its WYSIWYG content editor.
And it works very well for standard content pages.

A few months ago, the editor was implemented in the management of widgets.
And then, bang…

JS error when loading the page:


Uncaught (in promise) TypeError: e is undefined

And it becomes impossible to edit already configured sidebars:

While waiting for the Gutenberg issue to be resolved, you might want to switch back to the old widget editing mode.
For that, you just need to add this small directive to the functions.php file of your theme, or create a child theme if you're using a purchased theme that might get updated.

remove_theme_support( 'widgets-block-editor' );