crash when go in world

one of my worlds got destroyed(when i loaded it minecraft went back to main manu)
so i created a new super flat and after about 5 seconds when i start moving i crashed and got this:

---- Minecraft Crash Report ----
// You're mean.

Time: 2013-07-08 14:44
Description: Sending packet

java.lang.NullPointerException
	at cm.b(SourceFile:116)
	at cm.a(SourceFile:41)
	at jz.b(SourceFile:421)
	at hm.b(SourceFile:648)
	at hm.a(SourceFile:82)
	at bkz.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.t(SourceFile:488)
	at net.minecraft.server.MinecraftServer.s(SourceFile:405)
	at bkw.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:339)
	at hh.run(SourceFile:582)


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

-- Head --
Stacktrace:
	at cm.b(SourceFile:116)
	at cm.a(SourceFile:41)

-- Packet being sent --
Details:
	Packet ID: 4
	Packet class: fw
Stacktrace:
	at jz.b(SourceFile:421)
	at hm.b(SourceFile:648)
	at hm.a(SourceFile:82)
	at bkz.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.t(SourceFile:488)
	at net.minecraft.server.MinecraftServer.s(SourceFile:405)
	at bkw.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:339)
	at hh.run(SourceFile:582)