Exception in server tick loop : NullPointerException

I got this crash report I have never seen before :

Notes :
"Testure" is a world created around 1.4.
I was testing a custom texture for Coal Block when the crash happened but I don't think it's related since it's not the first Coal blocks I've placed ...
The texture pack I was using was vanilla. It only uses vanilla animation and is 16x.
I was using the last available version of the laucher (0.7.1)

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 06/05/13 07:24
Description: Exception in server tick loop

java.lang.NullPointerException
	at akg.f(SourceFile:197)
	at gw.m(SourceFile:689)
	at net.minecraft.server.MinecraftServer.a(SourceFile:880)
	at bha.a(SourceFile:199)
	at me.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.r(SourceFile:435)
	at bha.r(SourceFile:125)
	at net.minecraft.server.MinecraftServer.run(SourceFile:341)
	at gr.run(SourceFile:584)


A detailed walkthrough of the error, its code path and all known details is as follows:
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-- System Details --
Details:
	Minecraft Version: 13w18c
	Operating System: Windows XP (x86) version 5.1
	Java Version: 1.7.0_11, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 58742352 bytes (56 MB) / 273711104 bytes (261 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 8053 (450968 bytes; 0 MB) allocated, 7607 (425992 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: 1150 (64400 bytes; 0 MB) allocated, 1136 (63616 bytes; 0 MB) used
	Player Count: 1 / 8; [je['Missingno_26'/711, l='Testures', x=199,79, y=64,00, z=268,87]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.