Im in a snapshot 14w06b and when i try to open my singleplayer tab HELP PLZZZ!!!

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
I thought my minecraft would open.

What actually happened was...:
Describe what happened here
I open my singleplayer tab then it goes into a crash report.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2.Open minecraft.
3. open singleplayer