Posting an SSCCE helps a lot in providing context to your problem. You are asking us to guess what's null without code to show us what's null. Based upon the posts above, screen is instantiated in the constructor by calling getGraphicsConfiguration or maybe createVolatileImage (I'm confused about which given your code changes from post to post), I can guess
you are are receiving the exception because screen is null. To quote the java 6 API, which should be studied before using the methods contain within - for getGraphicsConfiguration():
If the Component has been created, but not yet added to a Container, this method returns null
and for createVolatileImage:
The return value may be null if the component is not displayable
Has the component been added to a container (given the instantiation of screen occurs in the constructor in may of your code snippets...I seriously doubt it)?