Crash after changing max view distance to 20 chunks in options.txt / Render distance too high in options.txt after downgrading

• Navigate to .minecraft/
• Open options.txt with an editor
• Change the line renderDistance:XX to something like renderDistance:12
• Save the file

Downgrading is not, was never and will never be officially supported. If you are using Snapshots, use them in a different folder like you are told to when activating Snapshots. Otherwise, issues like this will occur.