Hi,
It looks like we already have that patch in our side.
In the same file, please modify
$mail_subject = JText::sprintf($mail->subject, $order->order_number, $order->mail_status, HIKASHOP_LIVE);
to
$order_number = isset($order->order_number) ? $order->order_number : @$order->old->order_number;
$mail_subject = JText::sprintf($mail->subject, $order_number, $order->mail_status, HIKASHOP_LIVE);
We are currently working on HikaMarket 1.8.0, we hope to release it soon.
So all recent patches we made will be available.
Regards,