issue/81885 : throw an exception if the __wakeup() method is called

Author: stephane-gillot

src/ShoppingFeedCustomFields.php

@@ -97,5 +97,6 @@ private function __clone() {
 	 * Singleton instance can't be serialized.
 	 */
 	public function __wakeup() {
+		throw new \Exception( 'Cannot unserialize a singleton.' );
 	}
 }