Issue #3086397 by subson, kim.pepper: Registry does not declare a themeInitialization property

core/lib/Drupal/Core/Theme/Registry.php

@@ -134,6 +134,13 @@ class Registry implements DestructableInterface {
    */
   protected $themeHandler;
 
+  /**
+   * The theme initialization.
+   *
+   * @var \Drupal\Core\Theme\ThemeInitializationInterface
+   */
+  protected $themeInitialization;
+
   /**
    * The theme manager.
    *