Error message

• Warning: Cannot modify header information - headers already sent by (output started at /var/www/drupal/includes/bootstrap.inc:1640) in drupal_send_headers() (line 1499 of /var/www/drupal/includes/bootstrap.inc).
• Error: Class 'Drupal\form_builder\FormBase' not found in form_builder_cron() (line 360 of /var/www/drupal/sites/all/modules/contrib/form_builder/form_builder.module).