See
public Point2D localToScreen(double localX, double localY, double localZ) {
        Scene scene = getScene();
        Window window = scene.getWindow();
        if (scene == null || window == null) {
            return null;
        }

note testing scene==null *after* calling scene.getWindow().

public Point2D screenToLocal(double screenX, double screenY) {

has the same bug.