Currently the Damaged UI displays this error when an attempt is made to fetch a message that no longer exist in the database:
TypeError: Return value of CRM_Damaged_BAO_Damaged::retrieve() must be an instance of CRM_Damaged_BAO_Damaged, null returned in CRM_Damaged_BAO_Damaged::retrieve() (line 41 of /srv/org.wikimedia.civicrm/drupal/sites/default/civicrm/extensions/damaged/CRM/Damaged/BAO/Damaged.php).
We need to show a "Message not found" error instead.