This new Widgets editor expects <a href="https://wordpress.org/documentation/article/blocks-list/">WordPress blocks</a> instead of regular WordPress widgets. If you're using the classic WP Popular Posts widget please replace it with the <a href="https://cabrerahector.com/wordpress/wordpress-popular-posts-5-3-improved-php-8-support-retina-display-support-and-more/#block-editor-support">WP Popular Posts block</a> instead - it has the same features and functionality as the "classic" widget so you won't be missing anything at all. See the <a href="https://cabrerahector.com/wordpress/migrating-from-the-classic-popular-posts-widget/">Migration Guide</a> for more details.