Issue #3274474: Fix 'Access to an undefined property' PHPStan L0 errors

core/lib/Drupal/Core/Cache/Context/CacheContextsManager.php

@@ -36,6 +36,11 @@ class CacheContextsManager {
    */
   protected $contexts;
 
+  /**
+   * The set of valid context tokens.
+   */
+  protected array $validContextTokens;
+
   /**
    * Constructs a CacheContextsManager object.
    *