screen crash

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

Time: 4/12/13 8:08 AM
Description: Updating screen events

java.lang.RuntimeException: Failed to check session lock, aborting
	at ajt.h(SourceFile:45)
	at ajt.<init>(SourceFile:31)
	at ajp.<init>(SourceFile:16)
	at ajq.a(SourceFile:83)
	at net.minecraft.client.Minecraft.a(SourceFile:1363)
	at awo.a(SourceFile:204)
	at axr.a(SourceFile:72)
	at awo.a(SourceFile:305)
	at axr.d(SourceFile:127)
	at axr.m(SourceFile:111)
	at net.minecraft.client.Minecraft.l(SourceFile:1078)
	at net.minecraft.client.Minecraft.K(SourceFile:574)
	at net.minecraft.client.Minecraft.run(SourceFile:526)
	at java.lang.Thread.run(Unknown Source)


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

-- Head --
Stacktrace:
	at ajt.h(SourceFile:45)
	at ajt.<init>(SourceFile:31)
	at ajp.<init>(SourceFile:16)
	at ajq.a(SourceFile:83)
	at net.minecraft.client.Minecraft.a(SourceFile:1363)
	at awo.a(SourceFile:204)
	at axr.a(SourceFile:72)
	at awo.a(SourceFile:305)
	at axr.d(SourceFile:127)
	at axr.m(SourceFile:111)

-- Affected screen --
Details:
	Screen name: bkg
Stacktrace:
	at net.minecraft.client.Minecraft.l(SourceFile:1078)
	at net.minecraft.client.Minecraft.K(SourceFile:574)
	at net.minecraft.client.Minecraft.run(SourceFile:526)
	at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
	Minecraft Version: 1.5.1
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_15, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 472143192 bytes (450 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 2 total; -Xms512m -Xmx1024m
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	LWJGL: 2.4.2
	OpenGL: ATI Mobility Radeon HD 3650 GL version 3.3.11079 Compatibility Profile Context FireGL, ATI Technologies Inc.
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Texture Pack: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null