Run the attached sample:

        Circle node = new Circle(100, Color.RED);
        Scene scene = SceneBuilder.create().fill(Color.BLUE)
                .root(StackPaneBuilder.create().children(node).build()).build();
        Object renderToImage = scene.renderToImage(null);

The result is null. If I add sizes to scene with ".width(100).height(100)", it won't be null.