minecraft craches

Put the summary of the bug you're having here
iminecraft craches every time i loging on an older verision. my server is on 1.7.10
*What I expected to happen was to log in on my server regular but it just continues craches
Describe what you thought should happen here
log in to my server
*What actually happened was minecraft craches on an older verison
Describe what happened here minecraft cract wen i logedd in to minecraft 1.7.10 on my server

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2.i want you to fix the bug
3. ...