Error message

• Warning: Cannot modify header information - headers already sent by (output started at /home/www/ianjackson.id.au/includes/common.inc:2800) in drupal_send_headers() (line 1499 of /home/www/ianjackson.id.au/includes/bootstrap.inc).
• Error: Call to a member function get() on null in Drupal::service() (line 52 of /home/www/ianjackson.id.au/sites/all/modules/service_container/lib/Drupal.php).