Crash Report

I was just building a castle and this error just pops up with no warning;

---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.

Time: 11/16/12 8:00 PM
Description: Exception in server tick loop

java.lang.NullPointerException
	at mj.f(SourceFile:104)
	at mc.i(SourceFile:60)
	at is.a(SourceFile:89)
	at ii.a(SourceFile:96)
	at net.minecraft.server.MinecraftServer.r(SourceFile:759)
	at net.minecraft.server.MinecraftServer.q(SourceFile:686)
	at bdo.q(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:620)
	at fy.run(SourceFile:856)


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

-- System Details --
Details:
	Minecraft Version: 1.4.4
	Operating System: Windows XP (x86) version 5.1
	Java Version: 1.6.0_37, Sun Microsystems Inc.
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Sun Microsystems Inc.
	Memory: 380039816 bytes (362 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 2 total; -Xms512m -Xmx1024m
	AABB Pool Size: 2072 (116032 bytes; 0 MB) allocated, 1719 (96264 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 202 (11312 bytes; 0 MB) allocated, 187 (10472 bytes; 0 MB) used
	Player Count: 1 / 8; [iq['SilVERliGhT'/1755241, l='Nnormal superflat', x=1531.63, y=17.17, z=-517.45]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.