Crashing when trying to play a game.

Put the summary of the bug you're having here
Trying to play a single player game crashes Minecraft.
Describe what you thought should happen here
I expected the game to open and run smoothly.
Describe what happened here
What actually happened was that the game stopped loading, and crashed.
Steps to Reproduce:
1. Launch Minecraft.
2. Click on a single player game.
3. Wait while loading, then it crashes.