I'm trying to list all the rooms currently NOT completed, so I can make a WebScreen where a user can just click a link and join an ongoing conversation. 

For this, I created an action where the GetRooms action from the utils, with RoomName empty, and OnlyCompleted set to false, then aggregating the results to a list and returning that list. 

When I do it like this, I get a "Nullable object must have a value."

In the end, I could reach the same behavior through the REST API, but perhaps this should get fixed.