Solved NullPointerException by teleporting a player

I am making a plugin that teleport you in a different world. I made the gui and make when you click on slot 0 for exemple, it teleport you in a world but only the slot 4 can teleport me at a world and it work. For the others slots it give me an error. Here is the error : https://pastebin.com/2vwWS8ee
The main class : https://pastebin.com/xguVV6Em
and the GUI class : https://pastebin.com/4VrUa4qh
(Sorry for bad explanation I am French)

I solved the problem (happy ending)

EDIT by Moderator: merged posts, please use the edit button instead of double posting.