Hopper: java.lang.NullPointerException

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-84061/

Crash:

java.lang.NullPointerException
	at anq.f(SourceFile:197)
	at hw.m(SourceFile:685)
	at net.minecraft.server.MinecraftServer.a(SourceFile:866)
	at bnn.a(SourceFile:198)
	at nk.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.s(SourceFile:426)
	at bnn.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hr.run(SourceFile:575)

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

---- Minecraft Crash Report ----
// Don't be sad. I'll do better next time, I promise!

Time: 9/6/13 3:41 PM
Description: Exception in server tick loop

java.lang.NullPointerException
	at anq.f(SourceFile:197)
	at hw.m(SourceFile:685)
	at net.minecraft.server.MinecraftServer.a(SourceFile:866)
	at bnn.a(SourceFile:198)
	at nk.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.s(SourceFile:426)
	at bnn.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hr.run(SourceFile:575)


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

-- System Details --
Details:
	Minecraft Version: 13w36b
	Operating System: Windows Vista (x86) version 6.0
	Java Version: 1.7.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 168102264 bytes (160 MB) / 518979584 bytes (494 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 20081 (1124536 bytes; 1 MB) allocated, 1529 (85624 bytes; 0 MB) used
	IntCache: cache: 1, tcache: 1, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 1673 (93688 bytes; 0 MB) allocated, 228 (12768 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.

You can see this full report at http://hopper.minecraft.net/crashes/minecraft/MCX-84061/8417506/