Hi,
Thanks for the report we will see to remove this warning in the next release.
Please understand that it is not an error message but just a warning.
A warning is not critical at all and when you are in production ; you shouldn't display PHP warning like you should hide the PHP errors to just log them.
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.