exception:
java.lang.NullPointerException
        at org.apache.poi.hslf.model.Freeform.getOutline(Freeform.java:248)
        at org.apache.poi.hslf.model.ShapePainter.paint(ShapePainter.java:37)
        at org.apache.poi.hslf.model.TextShape.draw(TextShape.java:561)
        at org.apache.poi.hslf.model.Slide.draw(Slide.java:431)
code:

SlideShow ppt = new SlideShow(fromIS);

Dimension pgsize = ppt.getPageSize();
Slide[] slide = ppt.getSlides();
for (int i = 0; i < slide.length; i++) {

    BufferedImage img = new BufferedImage(pgsize.width, pgsize.height, BufferedImage.TYPE_INT_RGB);
    Graphics2D graphics = img.createGraphics();

    graphics.setPaint(Color.white);
    graphics.fill(new Rectangle2D.Float(0, 0, pgsize.width, pgsize.height));
    slide[i].draw(graphics);

    //save the output
    FileOutputStream out = new FileOutputStream(outputFile.getParent() +  "/slide-"  + (i+1) + ".png");
    javax.imageio.ImageIO.write(img, "png", out);
    out.close();
}