Error: java.lang.NullPointerException: Unexpected error

I try to start the game it self, from the launcher. It gets as far at the Mojang loading screen and shuts down. This is also the first time I got a chance to read the crash report as it never showed up before. it says Error: java.lang.NullPointerException: Unexpected error and I'm not sure how to fix it. Every now and then I am able to play without the error and the game loads up so I can choose single or multiplayer. Please help