Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/modules/field/field.post_update.php

@@ -0,0 +1,46 @@
+<?php
+
+/**
+ * @file
+ * Post update functions for Field module.
+ */
+
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\field\Entity\FieldConfig;
+
+/**
+ * @addtogroup updates-8.0.0-beta
+ * @{
+ */
+
+/**
+ * Re-save all field storage config objects to add 'custom_storage' property.
+ */
+function field_post_update_save_custom_storage_property() {
+  foreach (FieldStorageConfig::loadMultiple() as $field_storage_config) {
+    $field_storage_config->save();
+  }
+
+  return t('All field storage configuration objects re-saved.');
+}
+
+/**
+ * Fixes the 'handler' setting for entity reference fields.
+ */
+function field_post_update_entity_reference_handler_setting() {
+  foreach (FieldConfig::loadMultiple() as $field_config) {
+    $field_type_manager = \Drupal::service('plugin.manager.field.field_type');
+    $item_class = 'Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem';
+    $class = $field_type_manager->getPluginClass($field_config->getType());
+    if ($class === $item_class || is_subclass_of($class, $item_class)) {
+      // field_field_config_presave() will fix the 'handler' setting on save.
+      $field_config->save();
+    }
+  }
+
+  return t('Selection handler for entity reference fields have been adjusted.');
+}
+
+/**
+ * @} End of "addtogroup updates-8.0.0-beta".
+ */