Update to Drupal 8.2.6. For more information, see https://www.drupal.org/project/drupal/releases/8.2.6

vendor/symfony/validator/Util/PropertyPath.php

@@ -16,8 +16,6 @@ namespace Symfony\Component\Validator\Util;
  *
  * For more extensive functionality, use Symfony's PropertyAccess component.
  *
- * @since  2.5
- *
  * @author Bernhard Schussek <bschussek@gmail.com>
  */
 class PropertyPath
@@ -39,7 +37,7 @@ class PropertyPath
     public static function append($basePath, $subPath)
     {
         if ('' !== (string) $subPath) {
-            if ('[' === $subPath{0}) {
+            if ('[' === $subPath[0]) {
                 return $basePath.$subPath;
             }