[FIX] point_of_sale: Cancel payment when switching to floor plan

Payments that used terminals were not correctly canceled when switching
to the floor plan because there was no selected order. If a response was
received at that time, a traceback appeared.

X-original-commit: f9214eed9774dcddc57deb536dfcd23572972184

addons/point_of_sale/static/src/js/screens.js

@@ -2256,14 +2256,14 @@ var PaymentScreenWidget = ScreenWidget.extend({
     watch_order_changes: function() {
         var self = this;
         var order = this.pos.get_order();
-        if (!order) {
-            return;
-        }
         if(this.old_order){
             this.old_order.stop_electronic_payment();
             this.old_order.unbind(null, null, this);
             this.old_order.paymentlines.unbind(null, null, this);
         }
+        if (!order) {
+            return;
+        }
         order.bind('all',function(){
             self.order_changes();
         });