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

vendor/symfony/serializer/Encoder/XmlEncoder.php

@@ -461,6 +461,8 @@ class XmlEncoder extends SerializerAwareEncoder implements EncoderInterface, Dec
      * @param mixed    $val
      *
      * @return bool
+     *
+     * @throws UnexpectedValueException
      */
     private function selectNodeType(\DOMNode $node, $val)
     {
@@ -506,7 +508,7 @@ class XmlEncoder extends SerializerAwareEncoder implements EncoderInterface, Dec
     /**
      * Create a DOM document, taking serializer options into account.
      *
-     * @param array $context options that the encoder has access to.
+     * @param array $context options that the encoder has access to
      *
      * @return \DOMDocument
      */