Fix undefined property
Fix undefined property

It occurs when the cache can't be written.
felixgomez/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer,lucasmichot/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,felixgomez/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,keradus/PHP-CS-Fixer,Soullivaneuh/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer,Soullivaneuh/PHP-CS-Fixer,keradus/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,lucasmichot/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer