[FIX] website_slides: leave course
[FIX] website_slides: leave course

Currently there is a traceback when trying to leave a course (unsubscribe)
from the frontend on the homepage of a specific course. (when clicking on
"You're enrolled")
This fixes the traceback.

closes odoo/odoo#37384

Signed-off-by: Thibault Delavallee (tde) <2d325771bb6e8870d87f0f3f7b45589a8338e550@openerp.com>

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo