[FIX] website_sale: fallback on payment term

If `account.account_payment_term_immediate` doesn't exist, the `.id`
will crash.

opw-1841141

addons/website_sale/models/website.py

@@ -149,8 +149,11 @@ class Website(models.Model):
 
     @api.model
     def sale_get_payment_term(self, partner):
-        DEFAULT_PAYMENT_TERM = 'account.account_payment_term_immediate'
-        return partner.property_payment_term_id.id or self.env.ref(DEFAULT_PAYMENT_TERM, False).id
+        return (
+            partner.property_payment_term_id or
+            self.env.ref('account.account_payment_term_immediate', False) or
+            self.env['account.payment.term'].sudo().search([('company_id', '=', self.company_id.id)], limit=1)
+        ).id
 
     @api.multi
     def _prepare_sale_order_values(self, partner, pricelist):