Update to Drupal 8.1.5. For more information, see https://www.drupal.org/project/drupal/releases/8.1.5

core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php

@@ -30,8 +30,8 @@ trait DiscoveryTrait {
    * @param string $plugin_id
    *   A plugin id.
    * @param bool $exception_on_invalid
-   *   (optional) If TRUE, an invalid plugin ID will throw an exception.
-   *   Defaults to FALSE.
+   *   If TRUE, an invalid plugin ID will cause an exception to be thrown; if
+   *   FALSE, NULL will be returned.
    *
    * @return array|null
    *   A plugin definition, or NULL if the plugin ID is invalid and

core/lib/Drupal/Component/Plugin/composer.json

@@ -6,7 +6,7 @@
   "license": "GPL-2.0+",
   "require": {
     "php": ">=5.5.9",
-    "symfony/validator": "2.7.*"
+    "symfony/validator": "~2.7"
   },
   "autoload": {
     "psr-4": {