It happened to me once, it's easy to fix, you probably have in your mods folder a mod that isn't for 1.12.2. Remove it. If it doesn't help, try to reinstall Minecraft.
Nickname: noam15A Age: 10 Games: Minecraft, Minecraft and Minecraft. Discord: noam15A#9518 Reason I should be accepted: I dont think I should be accepted, I am doing this just to see if you will accept me. LOL. Recording Software: Bandicam full version