[FIX] website_event_sale: clear cart element in the session

Steps to reproduce:
-------------------
- create and pay with ewallet a cart with a ticket;
- create and pay with ewallet an other cart with a ticket;

Remark:
Don't clear element in the session like 'sale_order_id', etc.
For example, by moving the mouse over the cart or by making a payment process.

Issue:
------
An error is raised with the message:
"It is forbidden to modify a sales order which is not in draft status.".

Cause:
------
We apply an update to the cart on a sale order which is not in draft.

Solution:
---------
Make sure we are working on a sale order in draft
when applying a registration post.
If not, clear session elements.

opw-3465937

closes odoo/odoo#132832

Signed-off-by: Thomas Lefebvre (thle) <thle@odoo.com>

addons/website_event_sale/controllers/main.py

@@ -32,6 +32,9 @@ class WebsiteEventSaleController(WebsiteEventController):
             return super()._create_attendees_from_registration_post(event, registration_data)
 
         order_sudo = request.website.sale_get_order(force_create=True)
+        if order_sudo.state != 'draft':
+            request.website.sale_reset()
+            order_sudo = request.website.sale_get_order(force_create=True)
 
         tickets_data = defaultdict(int)
         for data in registration_data: