I was playing in creative when i typed "/effect add 21 10 10" and my Minecraft crashed instantly with "Loading entity NBT." Now every time I try to open this world (others work fine) my Minecraft crashes before the world even loads.

---- Minecraft Crash Report ----
// Everything's going to plan. No, really, that was supposed to happen.

Time: 5/27/13 10:32 AM
Description: Loading entity NBT

java.lang.NullPointerException
	at np.a(SourceFile:357)
	at tk.a(SourceFile:579)
	at jh.a(SourceFile:81)
	at mx.f(SourceFile:1061)
	at gz.a(SourceFile:152)
	at gz.a(SourceFile:52)
	at bie.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.s(SourceFile:491)
	at net.minecraft.server.MinecraftServer.r(SourceFile:408)
	at bib.r(SourceFile:125)
	at net.minecraft.server.MinecraftServer.run(SourceFile:342)
	at gu.run(SourceFile:585)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at np.a(SourceFile:357)
	at tk.a(SourceFile:579)
	at jh.a(SourceFile:81)

-- Entity being loaded --
Details:
	Entity Type: null (jh)
	Entity ID: 120
	Entity Name: Burbator
	Entity's Exact location: 1900.93, 63.00, -7088.57
	Entity's Block location: World: (1900,63,-7089), Chunk: (at 12,3,15 in 118,-444; contains blocks 1888,0,-7104 to 1903,255,-7089), Region: (3,-14; contains chunks 96,-448 to 127,-417, blocks 1536,0,-7168 to 2047,255,-6657)
	Entity's Momentum: 0.00, -0.02, 0.00
Stacktrace:
	at mx.f(SourceFile:1061)
	at gz.a(SourceFile:152)
	at gz.a(SourceFile:52)
	at bie.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.s(SourceFile:491)
	at net.minecraft.server.MinecraftServer.r(SourceFile:408)
	at bib.r(SourceFile:125)
	at net.minecraft.server.MinecraftServer.run(SourceFile:342)
	at gu.run(SourceFile:585)

-- System Details --
Details:
	Minecraft Version: 13w21b
	Operating System: Mac OS X (x86_64) version 10.7.4
	Java Version: 1.6.0_33, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 54963936 bytes (52 MB) / 113512448 bytes (108 MB) up to 1069416448 bytes (1019 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 916 (51296 bytes; 0 MB) allocated, 916 (51296 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 108 (6048 bytes; 0 MB) allocated, 108 (6048 bytes; 0 MB) used
	Player Count: 0 / 8; []
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.