model can be None (in case no effective model exists)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@28046 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/PaySheetTransaction.py

@@ -141,10 +141,12 @@ class PaySheetTransaction(Invoice):
     model = self.getSpecialiseValue().getEffectiveModel(\
         start_date=self.getStartDate(),
         stop_date=self.getStopDate())
+    sub_object_list = []
+    if model is not None:
+      # if there is an effective model
       model_reference_dict = model.getInheritanceReferenceDict(
                                      portal_type_list=portal_type_list,
                                      property_list=property_list)
-    sub_object_list = []
       traverse = self.getPortalObject().unrestrictedTraverse
       for model_url, id_list in model_reference_dict.items():
         model = traverse(model_url)