I’ve run into this issue myself when using a list view, I thought I was meant to construct the widget and add it myself to the list view but that’s actually not the case.
The item you are adding to the list is only an object supposed to contain information for the list view itself to create and setup the widget for you.
You can see an example here on how to use it: UE4 UMG ListView explained to my future self who has already forgotten... - YouTube