Hm, this could be due to UE4.8. I had similar problems when updating my Lobby System of the Marketplace from 4.7 to 4.8.
It seems like a lot of widgets are cleared when levels are changed.
This means, when the Server opens the new Server and the client looses its connection, it will return to the MainMenu and then through the Level change, the widgets will be destroyed. You could try to work around it (if this is the problem!) by check if the Variable is null and if yes, recreate the widget.
But this is a dirty way. Are you sure that the ShootOut example is not doing something else? :X It’s pretty hard to find this problem without debugging tbh.