I don't know what the solution is, but I can at least tell you that field_28090_g is empty, and that is the source of the NullPointerException. I don't know anything about making a new dimension, but I noticed that and thought it was worth mentioning.
So, I'm basically trying to [for starters] make a mod to add a Ball of Yarn that cats will play with. I haven't added any code for the cats to actually play with it, mostly because I can't figure out how to get it to render at all. In fact, I can't even get the icon for the item used to spawn it to render; it returns to the default iron helmet icon. Any help would be much appreciated. ModCatAdditions.java: ItemYarnBall.java: EntityYarnBall.java: RenderYarnBall.java: ModelYarnBall.java: CommonProxy.java: ClientProxy.java: Images: