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

vendor/symfony/validator/Constraints/NotNull.php

@@ -21,5 +21,11 @@ use Symfony\Component\Validator\Constraint;
  */
 class NotNull extends Constraint
 {
+    const IS_NULL_ERROR = 'ad32d13f-c3d4-423b-909a-857b961eb720';
+
+    protected static $errorNames = array(
+        self::IS_NULL_ERROR => 'IS_NULL_ERROR',
+    );
+
     public $message = 'This value should not be null.';
 }