fix: handle situations with no prev parent

Author: njamaleddine

polymorphic_tree/admin/parentadmin.py

@@ -189,10 +189,13 @@ def api_node_moved_view(self, request):
 
  position = request.POST['position']
 
- if isinstance(moved_id, integer_types) and isinstance(target_id, integer_types):
- previous_parent_id = int(request.POST['previous_parent_id']) or None
+ if request.POST.get('previous_parent_id'):
+ if isinstance(moved_id, integer_types) and isinstance(target_id, integer_types):
+ previous_parent_id = int(request.POST['previous_parent_id'])
+ else:
+ previous_parent_id = request.POST['previous_parent_id']
  else:
- previous_parent_id = request.POST['previous_parent_id'] or None
+ previous_parent_id = None
 
  # Not using .non_polymorphic() so all models are downcasted to the derived model.
  # This causes the signal below to be emitted from the proper class as well.