fixed warning with SystemObject cache

SE/se-lib/Schema/SystemObjectStorageAcl.php

@@ -302,6 +302,7 @@ class Schema_SystemObjectStorageAcl extends Core_AclSimpleSchemaBase {
 	}
 
 	public function clearGetItemCache($pk = null) {
+		if (!$this->_cache) return;
 		if (!$pk) $this->_cache = [];
 		else if (array_key_exists($pk, $this->_cache)) unset($this->_cache[$pk]);
 	}