Error message

• Notice: Undefined index: simplenews_block_form_8 in drupal_retrieve_form() (line 764 of /var/www/vhost/medspring.eu/home/html/includes/form.inc).
• Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'simplenews_block_form_8' not found or invalid function name in drupal_retrieve_form() (line 799 of /var/www/vhost/medspring.eu/home/html/includes/form.inc).