[MC-76717] Tried to load an old save, game crashed.

Put the summary of the bug you're having here

What I expected to happen was...:
I would successfully open an old single player world from a few versions ago.

What actually happened was...:
Load screen ensued and crashed after about a minute

Steps to Reproduce:
1. Have a world created a few versions ago?..
2. Try opening it via 1.8.1

Description: Rendering screen

java.lang.IllegalStateException: Already building!
	at ciu.a(SourceFile:114)
	at ciu.b(SourceFile:109)
	at bxe.c(SourceFile:452)
	at bxe.b_(SourceFile:440)
	at bxe.c(SourceFile:433)
	at bww.a(SourceFile:30)
	at cjh.b(SourceFile:1026)
	at bss.at(SourceFile:906)
	at bss.a(SourceFile:317)
	at net.minecraft.client.main.Main.main(SourceFile:124)