Well, that doesn't really answer my question. Judging from the log you've posted, somebody removes objects from the world. Why this fails in that case is another question, but the basic question is: Who? In jPCT itself, there's one line of code that does this and that inside the dispose() method of an Overlay. If you are using scaled blitting in the software renderer, an Overlay is internally used, which might be another source.
If you are not using any of these, i assume that your own code does this, which looks like a threading issue to me. The "normal" way to create a null pointer in renderScene() is to mess around with a world's set of objects while the world is being rendered.
If you are unsure about the removal, set the Logger to throw an Exception in case of an error. We should get the stack trace of that call then. Maybe that helps to find the source.