[FIX] point_of_sale: fixed issue in order to print the invoice report on POS  #318 #1270  

Fix : #318 #1270

**Generate Invoicing via POS**

***Impacted versions:***
- 8.0
- master

***Description :***

When we try to generate a invoice nothing happens. and there is an error in the browser console.

***How to reproduce :***
1. Go on runbot and pick up a v8.0 or a master instance
2. Sign in to the ERP (database using all modules)
3. Go to (point of sale) 
4. New Session or resume an old one
5. Add some products to the ticket and select a customer
6. Choose a payment method
7. Press invoice button

***Expected behaviour :***
The invoice button must display a generated PDF.

***Actual behaviour :***
Nothing is happened and the screen is frozen

Video: 
https://www.youtube.com/watch?v=VSzxyqqJexY&feature=youtu.be