The problem is that the "RokNavMenu - Boost" and "RokNavMenu - Extended Link" plugins added by your template seems to overwrite custom menu options. Which means that the product selection was missing from the menu options in the database. Then, when HikaShop try to load the menu options, it doesn't find that option which should always be there and thus that error.
I temporarily unpublished both plugins and saved the menu and it worked.
This product selection option is the only menu option in HikaShop. All the other menu's options are handled with our own system in the menu Display->Content menus, so you won't have problems with these two plugins if you display your HikaShop products as a listing of product or listing of categories menus but only with product page menus, in which case you can unpublish these plugins before creating the menus and then enabling them again.
We will change a bit the code of that view to avoid crashing when that happen.
Now, for the fact that the product selection is not saved, that process is handled automatically by joomla. HikaShop just defines a custom menu option to display the dropdown. So it must be a problem between these plugins and joomla.