Fall-back to default order to define new position

When there is no order on the screen, we must use the default order because it
contains the order of the relation field.

issue10298
review340621002

src/screen.js

@@ -1495,10 +1495,16 @@
             }.bind(this));
         },
         get new_position() {
-            if (this.order) {
-                for (var j = 0; j < this.order.length; j++) {
-                    var oexpr = this.order[j][0],
-                        otype = this.order[j][1];
+            var order;
+            if (this.order !== null) {
+                order = this.order;
+            } else {
+                order = this.default_order;
+            }
+            if (order) {
+                for (var j = 0; j < order.length; j++) {
+                    var oexpr = order[j][0],
+                        otype = order[j][1];
                     if ((oexpr == 'id') && otype) {
                         if (otype.startsWith('DESC')) {
                             return 0;