Crashes when i go to play the demo

Put the summary of the bug you're having here

What I expected to happen was...:
i was going to be able to play the demo version of minecraft

What actually happened was...:
game began to load up and than crashed

Steps to Reproduce:
1. log into mojang account on minecraft demo
2. click play demo world
3. came starts to load and than crashes almost a minute after it "building terrain comes up on screen