NullPointer While Opening Multiplayer Screen

While opening the Multiplayer menu, I suddenly get a violent crash saying:

java.lang.NullPointerException: Ticking screen
at gy.a(SourceFile:360)
at gr.m(SourceFile:195)
at gr.a(SourceFile:201)
(...etc)

I got the rest of the crash attached. I relaunched the game and can't seem to replicate it so I don't know what it was.