I've recently uninstalled the mod and whenever I try to create a new flat world the game crashes.
I thought that someone here could help me, so... here's the crash report.
crash-2018-08-02_01.48.58-server.txt

is it because I haven't uninstalled it properly?
thanks for the help.

Hello dude,
uninstall OTG-Core, it is in the mods folder inside of another folder called 1.12.2 (or whatever version you're using).

Cheers!

Caused by OTGCore still being present in the mods folder.