It could be any number of things. What did you do just before it happened? Did you possibly install an incompatible theme or app?
And please confirm that you did a battery pull - with the device on, you removed the battery for about 30 to 60 seconds, replaced it and let the BB reboot?
These errors are tough to pinpoint and it depends on what you were doing right before it happened. I copied your error code, "uncaught exception. java.lang.nullpointerexception", into Google and got a number of possibilities including the pn below. I suggest you give that a try and hopefully you can find an answer that works for you. Re: Uncaught exception: java.lang.NullPointerExcep... - BlackBerry Support Community Forums