Update to Drupal 8.1.0. For more information, see https://www.drupal.org/drupal-8.1.0-release-notes

vendor/symfony/validator/Constraints/IsNull.php

@@ -21,5 +21,11 @@ use Symfony\Component\Validator\Constraint;
  */
 class IsNull extends Constraint
 {
+    const NOT_NULL_ERROR = '60d2f30b-8cfa-4372-b155-9656634de120';
+
+    protected static $errorNames = array(
+        self::NOT_NULL_ERROR => 'NOT_NULL_ERROR',
+    );
+
     public $message = 'This value should be null.';
 }