[FIX] point_of_sale: fix traceback in pos_order studio

Current behavior:
If you try to edit the form view of pos.order in studio, you get a
traceback.

Steps to reproduce:
- Install point_of_sale
- Go to PoS order
- Click on studio
- Click on the button to edit the form view
- You get a traceback

opw-3008565

closes odoo/odoo#103838

X-original-commit: b616cad62a66aaaf8bfc7c79dbafd87174b9a1f6
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>

addons/point_of_sale/models/pos_order.py

@@ -366,6 +366,8 @@ class PosOrder(models.Model):
     @api.onchange('payment_ids', 'lines')
     def _onchange_amount_all(self):
         for order in self:
+            if not order.pricelist_id.currency_id:
+                raise UserError(_("You can't: create a pos order from the backend interface, or unset the pricelist, or create a pos.order in a python test with Form tool, or edit the form view in studio if no PoS order exist"))
             currency = order.pricelist_id.currency_id
             order.amount_paid = sum(payment.amount for payment in order.payment_ids)
             order.amount_return = sum(payment.amount < 0 and payment.amount or 0 for payment in order.payment_ids)