[FIX] pos_discount: error if misconfigured product

In case the discount product is misconfigured and therefore not loaded
by the POS, a traceback appears when applying a discount.

Add a comprehensive error message instead.

Closes #30574
opw-817527

closes odoo/odoo#30582

addons/pos_discount/i18n/pos_discount.pot

@@ -45,6 +45,13 @@ msgstr ""
 msgid "Discounts"
 msgstr ""
 
+#. module: pos_discount
+#. openerp-web
+#: code:addons/pos_discount/static/src/js/discount.js:28
+#, python-format
+msgid "No discount product found"
+msgstr ""
+
 #. module: pos_discount
 #: model:ir.model.fields,field_description:pos_discount.field_pos_config_iface_discount
 msgid "Order Discounts"
@@ -55,6 +62,13 @@ msgstr ""
 msgid "The default discount percentage"
 msgstr ""
 
+#. module: pos_discount
+#. openerp-web
+#: code:addons/pos_discount/static/src/js/discount.js:29
+#, python-format
+msgid "The discount product seems misconfigured. Make sure it is flagged as 'Can be Sold' and 'Available in Point of Sale'."
+msgstr ""
+
 #. module: pos_discount
 #: model:ir.model.fields,help:pos_discount.field_pos_config_discount_product_id
 msgid "The product used to model the discount"

addons/pos_discount/static/src/js/discount.js

@@ -23,6 +23,13 @@ var DiscountButton = screens.ActionButtonWidget.extend({
         var order    = this.pos.get_order();
         var lines    = order.get_orderlines();
         var product  = this.pos.db.get_product_by_id(this.pos.config.discount_product_id[0]);
+        if (product === undefined) {
+            this.gui.show_popup('error', {
+                title : _t("No discount product found"),
+                body  : _t("The discount product seems misconfigured. Make sure it is flagged as 'Can be Sold' and 'Available in Point of Sale'."),
+            });
+            return;
+        }
 
         // Remove existing discounts
         var i = 0;