Forge NullPointerException in injectSnapshot or MissingMappings When Loading World

What it looks like in the log

java.lang.NullPointerException: null @ [net.minecraftforge]

How to fix it

1. Make a full backup of your world folder immediately before running /fml confirm, as proceeding with a failed registry snapshot can permanently delete modded blocks and items.
2. Update Minecraft Forge to the latest recommended 1.12 or 1.12.2 build, since early 1.12 releases (such as 14.21.0.2324) had broken MissingMappings and snapshot injection logic.
3. If you are running SpongeForge on a server, update SpongeForge to a build explicitly targeting your installed Forge build number, or test without SpongeForge to confirm compatibility.
4. If this error appeared right after removing a mod, re-add that mod so the world can load cleanly, or update the remaining mods whose MissingMappings handlers appear in the stack trace.
5. If you are upgrading an older world from Minecraft 1.10 or earlier, load and save it in Forge 1.11.2 first rather than jumping directly to 1.12.

Check your own log