[FIX] event_sale: ticket

- Create a SO with an ticket for an event, confirm
- On the attendees wizard, click on 'Apply'

The event ticket is remove from the registration.

This is because the event ticket wasn't saved when creating the
registration line.

Complement of commits 7492c97b and 0717bb6c

opw-1920464

closes odoo/odoo#30262

addons/event_sale/wizard/event_edit_registration.xml

@@ -12,7 +12,7 @@
                         <tree string="Registration" editable="top" create="false" delete="false">
                             <field name="event_id" readonly='1' force_save="1"/>
                             <field name="registration_id" readonly='1' force_save="1"/>
-                            <field name="event_ticket_id" domain="[('event_id', '=', event_id)]" readonly='1'/>
+                            <field name="event_ticket_id" domain="[('event_id', '=', event_id)]" readonly='1' force_save="1"/>
                             <field name="name"/>
                             <field name="email"/>
                             <field name="phone"/>