Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/modules/contact/contact.post_update.php

@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * @file
+ * Post update functions for Contact.
+ */
+
+use Drupal\contact\Entity\ContactForm;
+
+/**
+ * @addtogroup updates-8.1.x-to-8.2.x
+ * @{
+ */
+
+/**
+ * Initialize 'message' and 'redirect' field values to 'contact_form' entities.
+ */
+function contact_post_update_add_message_redirect_field_to_contact_form() {
+  /** @var \Drupal\contact\ContactFormInterface $contact */
+  foreach (ContactForm::loadMultiple() as $contact) {
+    $contact
+      ->setMessage('Your message has been sent.')
+      ->setRedirectPath('')
+      ->save();
+  }
+}
+
+/**
+ * @} End of "addtogroup updates-8.1.x-to-8.2.x".
+ */