Salut j'ai un crash je n'arrive plus a jouer dans des serveurs

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

Time: 11/04/14 16:15
Description: Ticking screen

java.lang.NullPointerException: Ticking screen
at com.mojang.util.UUIDTypeAdapter.fromUUID(UUIDTypeA
dapter.java:22)

at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdap
ter.java:13)

at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdap
ter.java:10)

at com.google.gson.internal.bind.TypeAdapterRuntimeTy
peWrapper.write(TypeAdapterRuntimeTypeWrapper.java
:68)

at com.google.gson.internal.bind.ReflectiveTypeAdapte
rFactory$1.write(ReflectiveTypeAdapterFactory.java
:89)

at com.google.gson.internal.bind.ReflectiveTypeAdapte
rFactory$Adapter.write(ReflectiveTypeAdapterFactor
y.java:195)

at com.google.gson.Gson.toJson(Gson.java:593)
at com.google.gson.Gson.toJson(Gson.java:572)
at com.google.gson.Gson.toJson(Gson.java:527)
at com.google.gson.Gson.toJson(Gson.java:507)
at com.mojang.authlib.yggdrasil.YggdrasilAuthenticati
onService.makeRequest(YggdrasilAuthenticationServi
ce.java:52)

at com.mojang.authlib.yggdrasil.YggdrasilMinecraftSes
sionService.joinServer(YggdrasilMinecraftSessionSe
rvice.java:61)

at bll.a(SourceFile:48)
at jt.a(SourceFile:41)
at jt.a(SourceFile:10)
at ej.a(SourceFile:167)
at bct.c(SourceFile:78)
at ban.p(SourceFile:1343)
at ban.ak(SourceFile:774)
at ban.f(SourceFile:728)
at net.minecraft.client.main.Main.main(SourceFile:148
)

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

-- Head --
Stacktrace:
at com.mojang.util.UUIDTypeAdapter.fromUUID(UUIDTypeA
dapter.java:22)

at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdap
ter.java:13)

at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdap
ter.java:10)

at com.google.gson.internal.bind.TypeAdapterRuntimeTy
peWrapper.write(TypeAdapterRuntimeTypeWrapper.java
:68)

at com.google.gson.internal.bind.ReflectiveTypeAdapte
rFactory$1.write(ReflectiveTypeAdapterFactory.java
:89)

at com.google.gson.internal.bind.ReflectiveTypeAdapte
rFactory$Adapter.write(ReflectiveTypeAdapterFactor
y.java:195)

at com.google.gson.Gson.toJson(Gson.java:593)
at com.google.gson.Gson.toJson(Gson.java:572)
at com.google.gson.Gson.toJson(Gson.java:527)
at com.google.gson.Gson.toJson(Gson.java:507)
at com.mojang.authlib.yggdrasil.YggdrasilAuthenticati
onService.makeRequest(YggdrasilAuthenticationServi
ce.java:52)

at com.mojang.authlib.yggdrasil.YggdrasilMinecraftSes
sionService.joinServer(YggdrasilMinecraftSessionSe
rvice.java:61)

at bll.a(SourceFile:48)
at jt.a(SourceFile:41)
at jt.a(SourceFile:10)
at ej.a(SourceFile:167)
at bct.c(SourceFile:78)

-- Affected screen --
Details:
Screen name: bct
Stacktrace:
at ban.p(SourceFile:1343)
at ban.ak(SourceFile:774)
at ban.f(SourceFile:728)
at net.minecraft.client.main.Main.main(SourceFile:148
)

-- System Details --
Details:
Minecraft Version: 1.7.8
Operating System: Windows Vista (x86) version 6.0
Java Version: 1.7.0_40, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 48084584 bytes (45 MB) / 83505152 bytes (79 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerfor
mance_javaw.exe_minecraft.exe.heapdump
-Xmx512M
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 1.7.8
LWJGL: 2.9.1
OpenGL: GeForce G 103M/PCIe/SSE2 GL version 3.3.0, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Anisotropic filtering is supported and maximum anisotropy is 16.
Shaders are available because OpenGL 2.1 is supported.

Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: []
Current Language: Français (CA)
Profiler Position: N/A (disabled)
Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Anisotropic Filtering: Off (1)