Hi,
It is a simple warning, not a PHP (fatal) error.
And the warning is not in HikaSerial/HikaShop.
Please set the "error reporting" to "maximum" in your global Joomla configuration.
At this moment you should see the fatal error at the bottom of the page.
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.