Hopper: java.lang.NullPointerException

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-147845/

Crash:

java.lang.NullPointerException
	at aoc.f(SourceFile:197)
	at jg.m(SourceFile:685)
	at net.minecraft.server.MinecraftServer.a(SourceFile:868)
	at bnz.a(SourceFile:198)
	at nt.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.t(SourceFile:428)
	at bnz.t(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:334)
	at jb.run(SourceFile:577)

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

---- Minecraft Crash Report ----
// I just don't know what went wrong :(

Time: 9/13/13 5:02 PM
Description: Exception in server tick loop

java.lang.NullPointerException
	at aoc.f(SourceFile:197)
	at jg.m(SourceFile:685)
	at net.minecraft.server.MinecraftServer.a(SourceFile:868)
	at bnz.a(SourceFile:198)
	at nt.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.t(SourceFile:428)
	at bnz.t(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:334)
	at jb.run(SourceFile:577)


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

-- System Details --
Details:
	Minecraft Version: 13w37b
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_04, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 147753408 bytes (140 MB) / 396193792 bytes (377 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 7995 (447720 bytes; 0 MB) allocated, 1597 (89432 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 103
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 1698 (95088 bytes; 0 MB) allocated, 197 (11032 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-147845/10569493/