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-398271/

Crash:

java.lang.NullPointerException: Exception in server tick loop
	at na.d(SourceFile:103)
	at na.a(SourceFile:95)
	at mr.c(SourceFile:128)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at bsb.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at kx.run(SourceFile:617)

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

---- Minecraft Crash Report ----
// Don't be sad, have a hug! <3

Time: 10/18/13 12:33 PM
Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at na.d(SourceFile:103)
	at na.a(SourceFile:95)
	at mr.c(SourceFile:128)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at bsb.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at kx.run(SourceFile:617)


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

-- System Details --
Details:
	Minecraft Version: 13w42b
	Operating System: Windows XP (x86) version 5.1
	Java Version: 1.7.0_21, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 68810776 bytes (65 MB) / 351838208 bytes (335 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 1 total; -Xmx512M
	AABB Pool Size: 525 (29400 bytes; 0 MB) allocated, 368 (20608 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 44 (2464 bytes; 0 MB) allocated, 36 (2016 bytes; 0 MB) used
	Player Count: 1 / 8; [ml['AshJake'/16, l='New World', x=-9.76, y=237.00, z=34.77]]
	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-398271/19144115/