Issue #2959972 by donquixote: Undeclared property: ActiveTheme::$librariesExtend

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

@@ -82,6 +82,13 @@ class ActiveTheme {
    */
   protected $librariesOverride;
 
+  /**
+   * The list of libraries-extend definitions.
+   *
+   * @var array
+   */
+  protected $librariesExtend;
+
   /**
    * Constructs an ActiveTheme object.
    *