Hopper: java.lang.NullPointerException

java.lang.NullPointerException
	at nj.a(SourceFile:89)
	at of.aJ(SourceFile:384)
	at of.y(SourceFile:214)
	at og.y(SourceFile:128)
	at nn.l_(SourceFile:278)
	at of.l_(SourceFile:1149)
	at og.l_(SourceFile:169)
	at tm.l_(SourceFile:29)
	at abw.a(SourceFile:1349)
	at js.a(SourceFile:479)
	at abw.g(SourceFile:1327)

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 20.09.13 17:26
Description: Ticking entity

java.lang.NullPointerException
	at nj.a(SourceFile:89)
	at of.aJ(SourceFile:384)
	at of.y(SourceFile:214)
	at og.y(SourceFile:128)
	at nn.l_(SourceFile:278)
	at of.l_(SourceFile:1149)
	at og.l_(SourceFile:169)
	at tm.l_(SourceFile:29)
	at abw.a(SourceFile:1349)
	at js.a(SourceFile:479)
	at abw.g(SourceFile:1327)
	at abw.h(SourceFile:1234)
	at js.h(SourceFile:364)
	at net.minecraft.server.MinecraftServer.t(SourceFile:471)
	at net.minecraft.server.MinecraftServer.s(SourceFile:406)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)


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

-- Head --
Stacktrace:
	at nj.a(SourceFile:89)
	at of.aJ(SourceFile:384)
	at of.y(SourceFile:214)
	at og.y(SourceFile:128)
	at nn.l_(SourceFile:278)
	at of.l_(SourceFile:1149)
	at og.l_(SourceFile:169)
	at tm.l_(SourceFile:29)
	at abw.a(SourceFile:1349)
	at js.a(SourceFile:479)
	at abw.g(SourceFile:1327)

-- Entity being ticked --
Details:
	Entity Type: Skeleton (tr)
	Entity ID: 815926
	Entity Name: Skelett
	Entity's Exact location: 280.80, 49.00, 136.75
	Entity's Block location: World: (280,49,136), Chunk: (at 8,3,8 in 17,8; contains blocks 272,0,128 to 287,255,143), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Entity's Momentum: -0.02, -0.08, -0.06
Stacktrace:
	at abw.h(SourceFile:1234)
	at js.h(SourceFile:364)

-- Affected level --
Details:
	Level name: the ether
	All players: 0 total; []
	Chunk stats: ServerChunkCache: 625 Drop: 0
	Level seed: -861034799429911403
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (252,64,148), Chunk: (at 12,4,4 in 15,9; contains blocks 240,0,144 to 255,255,159), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 282179 game time, 494168 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 136321 (now: false), thunder time: 44964 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.t(SourceFile:471)
	at net.minecraft.server.MinecraftServer.s(SourceFile:406)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)

-- System Details --
Details:
	Minecraft Version: 1.6.4
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 22514200 bytes (21 MB) / 133672960 bytes (127 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 1995 (111720 bytes; 0 MB) allocated, 1995 (111720 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 1961 (109816 bytes; 0 MB) allocated, 1961 (109816 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.