I can't play multiplayer or start a single player world

Put the summary of the bug you're having here
I can't play multiplayer or start a single player world
What I expected to happen was...: for it to play normally
Describe what you thought should happen here
it to work
What actually happened was...:
Describe what happened here
it said it crashed
Steps to Reproduce:
1. my launcher says play offline start up
2. choose single player world crash
3. try multiplayer java.lang.illegalstateexception: failed to create a child event loop or it will say java.lang.nullpointerexception: group
4. also it says version 1.8.7