CG Maker: Exception: image == null! error

Wenn I’m trying to create a custom cg by using the cg maker I got this error:

Exception: image == null!
java.lang.IllegalArgumentException: image == null!
at javax.imageio.ImageTypeSpecifier.createFromRenderedImage(Unknown Source)
at javax.imageio.ImageIO.getWriter(Unknown Source)
at javax.imageio.ImageIO.write(Unknown Source)
at Graphics.CGImage.testWrite(
at Graphics.CGImage.main(

It appears right after it asks me where to save the cg. I have no clue how to fix it so I would be really happy if some one else have an answer for me.