Erro

Can someone help me and tell me why it’s doing this and possibly a solution?

Warning: Invalid argument supplied for foreach() in /var/www/kgs-lyngby.dk/public_html/wp-content/plugins/pushover-notifications/includes/notification-functions.php on line 368

Warning: Cannot modify header information – headers already sent by (output started at /var/www/kgs-lyngby.dk/public_html/wp-content/plugins/pushover-notifications/includes/notification-functions.php:368) in /var/www/kgs-lyngby.dk/public_html/wp-admin/post.php on line 231