PHP Deprecated: html_entity_decode(): Passing null to parameter #1 ($string) of type string

The line in question is:

$desc = html_entity_decode( $item->get_description(), ENT_QUOTES, get_option( 'blog_charset' ) );

Which heavily implies that a widget in your site, one that’s rarely used, has a null value for its description, or doesn’t bother to define one.

You will need to search for such a widget. It may be that it does not call the parent constructor in its constructor to pass it along.