Hopper: java.lang.NullPointerException: Exception in server tick loop

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-317304/

Crash:

java.lang.NullPointerException: Exception in server tick loop
	at mu.d(SourceFile:103)
	at mu.a(SourceFile:95)
	at ml.c(SourceFile:128)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at brh.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at kt.run(SourceFile:617)

The following is the first clean report made public about this crash:

---- Minecraft Crash Report ----
// Why did you do that?

Time: 10/11/13 8:14 PM
Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at mu.d(SourceFile:103)
	at mu.a(SourceFile:95)
	at ml.c(SourceFile:128)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at brh.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at kt.run(SourceFile:617)


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

-- System Details --
Details:
	Minecraft Version: 13w41b
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.6.0_22, Sun Microsystems Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
	Memory: 72533312 bytes (69 MB) / 583794688 bytes (556 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
	AABB Pool Size: 1989 (111384 bytes; 0 MB) allocated, 661 (37016 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 157 (8792 bytes; 0 MB) allocated, 65 (3640 bytes; 0 MB) used
	Player Count: 2 / 8; [mf['johndeathbringer'/39, l='Supper Flat Survival', x=74.69, y=36.71, z=148.26], mf['vinpainbringer'/5382831, l='Supper Flat Survival', x=39.09, y=1.00, z=152.43]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

You can see this full report at http://hopper.minecraft.net/crashes/minecraft/MCX-317304/17000136/