Fix: creation of dynamic property deprecation.

Closes #3520436

tests/src/Functional/KeyOverrideAdminTest.php

@@ -20,6 +20,13 @@ class KeyOverrideAdminTest extends BrowserTestBase {
    */
   protected $defaultTheme = 'stark';
 
+  /**
+   * The Drupal user entity.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $overrideUser;
+
   /**
    * {@inheritdoc}
    */