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

tryton/gui/window/view_form/screen/screen.py

@@ -635,8 +635,12 @@ class Screen(SignalEvent):
 
     @property
     def new_position(self):
-        if self.order:
-            for oexpr, otype in self.order:
+        if self.order is not None:
+            order = self.order
+        else:
+            order = self.default_order
+        if order:
+            for oexpr, otype in order:
                 if oexpr == 'id' and otype:
                     if otype.startswith('DESC'):
                         return 0