Hi,
In the plugin, please remove the line
in line 207.
That line is currently useless, the real call of the function is made few lines below and have the $user_id parameter which is expected.
I saw that it is already solved for HikaShop 2.6.1 but we will update the HikaShop 2.6.0 package with the patch as soon as possible.
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.