Merge branch 'bugfix/role-singals' into 'master'

Fix bug in signals for roles

See merge request !147

greg/signals.py

@@ -133,7 +133,7 @@ def save_notification_callback(sender, instance, created, *args, **kwargs):
     if not isinstance(instance, SUPPORTED_MODELS):
         return
     # Queue future notifications on start and end date for roles
-    if isinstance(instance, Role):
+    if isinstance(instance, Role) and hasattr(instance, "_changed_fields"):
         if (
             "start_date" in instance._changed_fields  # pylint: disable=protected-access
             and instance.start_date