Cannot assign default values for UserWidget variable in blueprints

Cannot assign default values for UserWidget variable. This limits their usability in blueprint classes, and really prevents them from being useful in UserStructs, particularly if that user struct is used as a payload for a Sequencer event track.

1. Create a Widget Blueprint asset
2. Create a new blueprint and add a variable of type User Widget (Reference, Class, Asset ID, Class Asset ID, try all options)
3. Compile the blueprint
4. In the variable's defaults, try to assign the Widget Blueprint class you created to the User Widget variable
5. Notice in the asset picker window, there are no assets displayed, so nothing can be assigned to a variable of User Widget type in the defaults