The world hangs forever on load in single player after upgrading from forge 12.18.1.2061 to 12.18.1.2063. This is with ElecCore-1.9.4-1.5.303. It worked for sure at least once after upgrading, maybe twice? After which it would just hang everytime.

Backdating to forge 2061 resolves the issue.

The part from latest.log
http://pastebin.com/AZGp2tnG

Can I please have the entire log?

I got the same error. Here's the full log: https://gist.github.com/mezz/d60afe2ba1e4d647cb2831b565faa6dc
Something in my dev environment crashed while I was loading a world, and then the eleccore onWorldSave handler crashed and caused the game to hang forever.

Because of this: http://prntscr.com/cndsx8, MC crashes before a WorldLoadEvent has been fired.
This causes a NBT tag to be null when the WorldUnloadEvent gets fired, which causes a NPE.

Closing as the root cause is not ElecCore

I believe ElecCore will cause the game to hang when any mod crashes during world load. Please protect against this case, it makes it much harder to figure out what's actually crashing.

I will add a protection for this

Seems like I already did that a few versions ago :P
00e5f4f