When I click to open a world, the game instantly crashes.

Put the summary of the bug you're having here

What I expected to happen was...:

I expected the world I was working on to open normally
What actually happened was...:

Yet it crashed before opening. No visuals, just open and crash.
Steps to Reproduce:
1. Launch Minecraft
2. Select "Play Offline
3. Single Player
4. Select a World
5. Loading World
6. Building terrain
7. Crash