Error message

• Warning: Cannot modify header information - headers already sent by (output started at /mnt/persist/www/docroot/includes/common.inc:2791) in drupal_send_headers() (line 1499 of /mnt/persist/www/docroot/includes/bootstrap.inc).
• TypeError: Argument 1 passed to xmlsitemap_node_create_link() must be an instance of stdClass, boolean given, called in /mnt/persist/www/docroot/sites/all/modules/contrib/xmlsitemap/xmlsitemap_node/xmlsitemap_node.module on line 28 in xmlsitemap_node_create_link() (line 194 of /mnt/persist/www/docroot/sites/all/modules/contrib/xmlsitemap/xmlsitemap_node/xmlsitemap_node.module).