Crash on loading world files, server and on disk

Put the summary of the bug you're having here

What I expected to happen was...:
to load up a world file from a server smoothly

What actually happened was...:
the game crashed as the map was loaded from ram

Steps to Reproduce:
1. open the launcher
2. click the custom 1.7.10 minecraft instance for a specific server
3. try to open the server