[FIX] point_of_sale: fix doubleclick of validate button

Open POS, add something to the order, go to checkout, add payment,
double click on validate

js error will popup

opw-2429523

closes odoo/odoo#66571

X-original-commit: c3aea6783b8bfb3af383ae881af247fa5507e99f
Signed-off-by: agr-odoo <agr-odoo@users.noreply.github.com>

addons/point_of_sale/static/src/js/models.js

@@ -946,8 +946,12 @@ exports.PosModel = Backbone.Model.extend({
 
         return new Promise(function (resolve, reject) {
             self.flush_mutex.exec(function () {
-                var flushed = self._flush_orders([self.db.get_order(order_id)], opts);
-
+                var order = self.db.get_order(order_id);
+                if (order){
+                    var flushed = self._flush_orders([order], opts);
+                } else {
+                    var flushed = Promise.resolve([]);
+                }
                 flushed.then(resolve, reject);
 
                 return flushed;