Resolve #3401128 "1"

Closes #3401128

src/Annotation/MarkdownExtension.php

@@ -19,5 +19,6 @@ class MarkdownExtension extends InstallablePlugin {
    * @see https://www.drupal.org/project/markdown/issues/3142418
    */
   public $requires = [];
+  public $_requiredBy = [];
 
 }