Blank PDF out put after upgrade of Hikashop

-- HikaShop version -- : 3.4.1
-- Joomla version -- : 3.8.10
-- PHP version -- : 7.1

Hello,

I use Hikashop as a brochure, no actual sales. I have added a custom PDF generator of the product page which has worked fine for a year.
I decided it was time to update my Joomla to the latest version and this seemed to break the category links in my Hikashop.
So I updated Hikashop and the links are resolved, but now my custom PDF generator does not work - it shows a blank page

Now I know there will be no support for a custom add-on. But I see that HikaShop 3.2.2 included "We've updated the HTML2PDf and TCPDF libraries of the PDF invoice plugin to the latest versions."

I guess this is the problem. So I am trying to find these changes and change them back to how they were. Any tips on where to look for this?

To confirm:

1. PDF works with Hikashop 2.6.3 and Joomla 3.6.5


2. Update Joomla to 3.8.10 and PDF output still works, but there are link problems in Hikashop


3. Update Hikashop to latest version and links are fixed but PDF output shows blank

Thanks for the reply - thats awesome. I fixed it with the patch.