It is extremely hard to debug FXML files.

Sometimes you suddenly receive something like
Caused by: javafx.fxml.LoadException: Element does not define a default property.

What element? At what row? At what character? What is expected?

IMHO such exceptions should be descriptive enough to be able to point to the error point as close as possible.

As for now, it just says "You have an error somewhere there".

I propose to put more details in all fxml loading exceptions.