My launcher loads and works to play Minecraft but as soon as it loads the Mojang syble it chrashes and goes back to the launcher

Put the summary of the bug you're having here

What I expected to happen was...:
it to load up Minecraft
What actually happened was...:
It crashed and went back to the launcher

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