Caused by: java.lang.NullPointerException
at javafx.scene.control.DialogPane$8.invalidated(DialogPane.java:627)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:109)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:143)
at javafx.scene.control.DialogPane.setExpanded(DialogPane.java:655)
... 116 more

The use case is SceneBuilder where one can create a layout with a DialogPane: the expanded property can be set while no expandableContent is there.