Hi,
That's not really error messages.
These are messages for developers to notify them of code that will need to be updated to continue working in future versions of PHP.
Normally users should not see them.
If you turn off the error reporting / debug settings of the Joomla configuration, you won't see them.
To remove them, you can try changing the code:
if(!isset($infoGet) || count($infoGet) == 0 || (empty($infoGet[0]) && !strlen($infoGet[0])))
to:
if(!isset($infoGet) || count($infoGet) == 0 || !isset($infoGet[0]) || (empty($infoGet[0]) && !strlen($infoGet[0])))
in the file /administrator/components/com_hikashop/classes/filter.php
Let us know how it goes so that we can include the change on our end.