Error message

• Warning: Cannot modify header information - headers already sent by (output started at /app/web/includes/common.inc:2797) in drupal_send_headers() (line 1499 of /app/web/includes/bootstrap.inc).
• Error: Class 'Drupal\form_builder\FormBase' not found in form_builder_cron() (line 360 of /app/web/sites/all/modules/form_builder/form_builder.module).