Cannot open Scene Builder in IntelliJ for MaterialFX demo project

Question

I have downloaded the following project to see how the demo application works:

https://github.com/palexdev/MaterialFX

I can successfully build and run it, I can use the demo app, and see all the features it has.

The problem is when I want to edit the .fxml files with the built-in Scene Builder in IntelliJ:

What and how should I set it up to be able to edit with it? I think I could edit it with the separate Scene Builder app but I would prefer to do it with the built-in one.

Cheers!