Minecraft will not let me open worlds!

At first I was just mining, and then I started to have what seemed to be connection lag, and then it crashed. I tried opening my world again, and I seemed to have the same problem, but it crashed much faster. Now my worlds will just load for a bit then it will crash.

Crash Report:

---- Minecraft Crash Report ----
// This is a token for 1 free hug. Redeem at your nearest Mojangsta: [~~HUG~~]

Time: 2/12/13 8:33 PM
Description: Exception in server tick loop

java.lang.NullPointerException
	at cg.b(SourceFile:103)
	at cg.a(SourceFile:35)
	at jh.b(SourceFile:429)
	at gu.a(SourceFile:78)
	at bil.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.r(SourceFile:480)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at bii.q(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:567)


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

-- System Details --
Details:
	Minecraft Version: 13w06a
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_07, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 210253264 bytes (200 MB) / 607694848 bytes (579 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 2 total; -Xms512m -Xmx1024m
	AABB Pool Size: 312766 (17514896 bytes; 16 MB) allocated, 312766 (17514896 bytes; 16 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: 265445 (14864920 bytes; 14 MB) allocated, 265445 (14864920 bytes; 14 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.