Error message

• Warning: Cannot modify header information - headers already sent by (output started at /var/www/ielts-asia-2018/html/includes/common.inc:2861) in drupal_send_headers() (line 1551 of /var/www/ielts-asia-2018/html/includes/bootstrap.inc).
• Error: Class 'Drupal\form_builder\FormBase' not found in form_builder_cron() (line 360 of /var/www/ielts-asia-2018/html/sites/all/modules/form_builder/form_builder.module).
• Warning: array_keys() expects parameter 1 to be array, null given in template_preprocess_maintenance_page() (line 2871 of /var/www/ielts-asia-2018/html/includes/theme.inc).
• Warning: Invalid argument supplied for foreach() in template_preprocess_maintenance_page() (line 2871 of /var/www/ielts-asia-2018/html/includes/theme.inc).