https://api.drupal.org/api/drupal/core%21modules%21system%21tests%21modules%21entity_test%21src%21Plugin%21Validation%21Constraint%21TestValidatedReferenceConstraint.php/property/TestValidatedReferenceConstraint%3A%3Amessage/10
TestValidatedReferenceConstraint::$message | Drupal API
messagedrupalapi