fixed issue when validating study subject form in edit appointment view

smash/web/views/appointment.py

@@ -103,7 +103,8 @@ def appointment_edit(request, id):
                                                status=Appointment.APPOINTMENT_STATUS_FINISHED).count() == 0:
                 adjust_date = True
             if appointment_form.cleaned_data["status"] == Appointment.APPOINTMENT_STATUS_FINISHED:
-                if not study.check_nd_number(study_subject_form.cleaned_data["nd_number"]):
+                nd_number = study_subject_form.cleaned_data["nd_number"]
+                if (nd_number is not None or nd_number != "") and not study.check_nd_number(nd_number):
                     study_subject_form.add_error('nd_number', ValidationError("invalid ND number"))
                     is_valid_form = False
         if is_valid_form: