Create widget from variable in child BPs?

My goal is to have a base blueprint that creates a widget and does stuff to it (positioning, etc.). But I want to have children of that base BP define what widget type to create. Here is what I have:

“Widget Class” is a variable of type Object that I set to a widget blueprint asset I choose from the content browser with base class User Widget. Sadly the cast is failing, so I must be missing something obvious.

Any help is appreciated.