goes to load world flashing "not responding" task then crash

Put the summary of the bug you're having here

What I expected to happen was...:
The game should load the world

What actually happened was...:
got to generating terrain then crashed

Steps to Reproduce:
1. Load Launcher
2. Play offline
3. loads game ok
4. choose single player
5. choose world
6. loads as far as generating terrain then crashes