ProgressIndicator is kind of static value - user cannot change it. So far the policy was that only widgets that are editable have Value.
The reason is that if you want to store all user supplied values in the dialog, you can simply go through all widgets and store the value if it is not Value::Void - that indicates that the widget does not have associated value.
but i'd prefer to have them all be accessible via the GetSetData as well, and in case of restoring sth..just make sure that exceptions are reinitialized well after restoring..