Cannot Preview a Node in a UI Material

Previewing a Node in a UI Material will not show anything, or will remain showing what is placed in Final Color.

This issue appears to be fixed in Dev-Main as of CL-2914624.

1. Open a New Blank Project with No Starter Content
2. Create a New UI Material
3. Add a Vector 3 set to 1,0,0
4. Right Click the Vector 3 Node
5. Preview Node

RESULTS: Nothing Previews

EXPECTED: Preview Red Node

[Repro] 'Preview Material Node doesn't work with UI Materials