Same problem here: Warning: require_once(/srv/htdocs/wp-content/plugins/add-search-to-menu/includes/class-is-stemmer.php): failed to open stream: No such file or directory in /srv/htdocs/wp-content/plugins/add-search-to-menu/add-search-to-menu.php on line 151
Fatal error: require_once(): Failed opening required ‘/srv/htdocs/wp-content/plugins/add-search-to-menu/includes/class-is-stemmer.php’ (include_path=’.:/usr/local/php7.3/lib/php’) in /srv/htdocs/wp-content/plugins/add-search-to-menu/add-search-to-menu.php on line151