Hello,
It means that your template do not include an HTML element which is required by the javascript.
So when the JS try to access that element to change the "class", it do not retrieve it and you got the error.
In order to fix it, it would require to know what is the concerned element. Unfortunately with just the message it do not allow to determine that. It needs more details (JS line, etc).
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.