Product creation error

-- url of the page with the problem -- : www.associazioneformatori.it
-- HikaShop version -- : 3.2.2
-- Joomla version -- : 3.8.4
-- PHP version -- : 7.1

Hi,
when I create a product and save it, I receive this error message: Call to a member function get() on boolean
After that I check the database and I find a product without category.
I find it in the "radice", so I open it and assign the category again to save it without error.