minecraft version 1.7.9 crashes when entering sever/new world/existing world/. i entered a sever named The Nexus.this server was DESIGNEd to run on 1.7.9

Put the summary of the bug you're having here

What I expected to happen was...:
open the server normally

What actually happened was...:
minecraft CRASHED, and opened the launcher

Steps to Reproduce:
1. open minecraft lancher
2. use minecraft 1.7,9
3. open world/server