Hello,
It is a PHP notice and not an error.
Note that notices should not be displayed in a production website.
Regarding the source of the issue ; it comes from a view override that you have, so it's not in the official HikaShop file.
I don't know why you have customized your backend order listing view, but that override now generate a notice.
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.