Hopper: java.lang.NullPointerException

java.lang.NullPointerException
	at java.util.HashSet.iterator(HashSet.java:153)
	at ki.a(SourceFile:229)
	at ki.b(SourceFile:235)
	at ki.a(SourceFile:205)
	at jx.a(SourceFile:125)
	at net.minecraft.server.MinecraftServer.t(SourceFile:471)
	at net.minecraft.server.MinecraftServer.s(SourceFile:398)
	at bng.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hs.run(SourceFile:575)

---- Minecraft Crash Report ----
// Why is it breaking :(

Time: 9/5/13 5:47 PM
Description: Exception in server tick loop

java.lang.NullPointerException
	at java.util.HashSet.iterator(HashSet.java:153)
	at ki.a(SourceFile:229)
	at ki.b(SourceFile:235)
	at ki.a(SourceFile:205)
	at jx.a(SourceFile:125)
	at net.minecraft.server.MinecraftServer.t(SourceFile:471)
	at net.minecraft.server.MinecraftServer.s(SourceFile:398)
	at bng.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hs.run(SourceFile:575)


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

-- System Details --
Details:
	Minecraft Version: 13w36a
	Operating System: Mac OS X (x86_64) version 10.6.8
	Java Version: 1.6.0_51, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 43049584 bytes (41 MB) / 187019264 bytes (178 MB) up to 1069416448 bytes (1019 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 7703 (431368 bytes; 0 MB) allocated, 5832 (326592 bytes; 0 MB) used
	Suspicious classes: [com.ibm.icu.text.ArabicShapingException], [gnu.trove.TIntCollection], [gnu.trove.impl.HashFunctions, PrimeFinder], [gnu.trove.impl.hash.THash, TPrimitiveHash, TIntIntHash], [gnu.trove.iterator.TIterator, TAdvancingIterator, TIntIntIterator], [gnu.trove.map.TIntIntMap], [gnu.trove.map.hash.TIntIntHashMap], [gnu.trove.procedure.TIntIntProcedure], [gnu.trove.set.TIntSet]
	IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 1735 (97160 bytes; 0 MB) allocated, 1424 (79744 bytes; 0 MB) used
	Player Count: 1 / 8; [kg['cblack4'/249, l='New World---', x=219.50, y=80.00, z=251.50]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.