Crash when trying to start the game

What I expected to happen was...:
Describe what you thought should happen here
After downloading and playing the 14w31a version (single player) I wanted to select the 1.7.10 version to play online.

What actually happened was...:
Describe what happened here
On the 1.7.10 version, the games starts but when trying to connect to a server or open a single player save, the game crashes.

Steps to Reproduce:
1. Download and play the 14w31a version
2. close the game and the launcher
3. Select the earlier (1.7.10) version
4. Start the game
5. Select single or multiplayer and connect to an offline save or to a server.