diff --git a/core/lib/Drupal/Core/Config/DatabaseStorage.php b/core/lib/Drupal/Core/Config/DatabaseStorage.php
index b017e9b..70096a0 100644
--- a/core/lib/Drupal/Core/Config/DatabaseStorage.php
+++ b/core/lib/Drupal/Core/Config/DatabaseStorage.php
@@ -186,7 +186,7 @@ protected function ensureTableExists()  {
   /**
    * Defines the schema for the configuration table.
    */
-  public static function schemaDefinition() {
+  protected static function schemaDefinition() {
     $schema = array(
       'description' => 'The base table for configuration data.',
       'fields' => array(
diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
index 6627b3d..2580ae4 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -328,12 +328,14 @@ public function preSave(EntityStorageInterface $storage) {
 
     // Ensure this entity's UUID does not exist with a different ID, regardless
     // of whether it's new or updated.
-    $matching_entities = $storage->getQuery()
-      ->condition('uuid', $this->uuid())
-      ->execute();
-    $matched_entity = reset($matching_entities);
-    if (!empty($matched_entity) && ($matched_entity != $this->id()) && $matched_entity != $this->getOriginalId()) {
-      throw new ConfigDuplicateUUIDException("Attempt to save a configuration entity '{$this->id()}' with UUID '{$this->uuid()}' when this UUID is already used for '$matched_entity'");
+    if ($uuid = $this->uuid()) {
+      $matching_entities = $storage->getQuery()
+        ->condition('uuid', $this->uuid())
+        ->execute();
+      $matched_entity = reset($matching_entities);
+      if (!empty($matched_entity) && ($matched_entity != $this->id()) && $matched_entity != $this->getOriginalId()) {
+        throw new ConfigDuplicateUUIDException("Attempt to save a configuration entity '{$this->id()}' with UUID '{$this->uuid()}' when this UUID is already used for '$matched_entity'");
+      }
     }
 
     // If this entity is not new, load the original entity for comparison.
diff --git a/core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php b/core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php
index 3d2575b..3a19370 100644
--- a/core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php
+++ b/core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php
@@ -178,7 +178,7 @@ public function setIfNotExists($key, $value) {
     }
     // Now that the bin has been created, try again if necessary.
     if ($try_again) {
-      $this->setIfNotExists($key, $value);
+      return $this->setIfNotExists($key, $value);
     }
   }
 
@@ -273,7 +273,7 @@ protected function catchException(\Exception $e) {
   /**
    * Defines the schema for the batch table.
    */
-  public static function schemaDefinition() {
+  public function schemaDefinition() {
     return [
       'description' => 'Generic key-value storage table. See the state system for an example.',
       'fields' => [
diff --git a/core/modules/system/src/Tests/Entity/FieldWidgetConstraintValidatorTest.php b/core/modules/system/src/Tests/Entity/FieldWidgetConstraintValidatorTest.php
index 4f223e3..f9dec1f 100644
--- a/core/modules/system/src/Tests/Entity/FieldWidgetConstraintValidatorTest.php
+++ b/core/modules/system/src/Tests/Entity/FieldWidgetConstraintValidatorTest.php
@@ -29,7 +29,7 @@ class FieldWidgetConstraintValidatorTest extends KernelTestBase {
   protected function setUp() {
     parent::setUp();
 
-    $this->installSchema('system', ['router', 'key_value']);
+    $this->installSchema('system', ['router']);
     $this->container->get('router.builder')->rebuild();
 
     $this->installEntitySchema('user');