Hello,
You can replace the line with :
if( (!empty($order->order_type) && $order->order_type != 'subsale') || (!empty($order->old) && $order->old->order_type != 'subsale') )
And it should avoid the warning.
Please note that you should not display PHP warning in a production environment.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.