Hi,
The notice message will be fix in the next release.
It is just a PHP notice, it is not seen as an error by PHP ; this kind of message can be display in a test/dev environment but should be hidden in a production mode.
If you want to fix that message, you can edit the plugin and replace in the line 62, the content
By
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.