Cant join Minecraft servers

I used to be able to connect to servers and then one day I tried going into mineplex and it said java.lang.NullPointerException: group I thought the server might be down so I tried multiple other servers and the same thing happened. The other night I randomly tried to go on Mineplex again and I got in I played for a while and then the next day the same thing happened. I looked at multiple tutorials on how to fix this and nothing helped, I think it's something to do with java but I don't know. Btw I don't run any mods or plugins.