Do you get the same error when you set things up like in the screenshot above?
You assign a specific animation instead of using an animation blueprint? You probably want to be using animation blueprints for animating your skeletal meshes.
When you make a duplicate of a widget (or of any actor), any code that is referencing the old object will not be able to simply access the old object the same way. With regular Objects and Actors you can create a child actor to get around some of this, but not with widgets. You will have to go through any place that references the old widget type (using FindInBlueprints, likely) and set them to use your new widget. Alternatively you can just modify the original one directly.