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Me and my brother are creating a Hardcore World, however this happens whenever we try to log on each other's worlds (Note: We use the same version with Optifine)
Description: Ticking screen

java.lang.NullPointerException: Ticking screen
at com.mojang.util.UUIDTypeAdapter.fromUUID(UUIDTypeAdapter.java:22)
at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdapter.java:13)
at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdapter.java:10)
at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.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.YggdrasilAuthenticationService.makeRequest(YggdrasilAuthenticationService.java:54)
at com.mojang.authlib.yggdrasil.YggdrasilMinecraftSessionService.joinServer(YggdrasilMinecraftSessionService.java:61)
at bll.a(SourceFile:48)
at jt.a(SourceFile:41)
at jt.a(SourceFile:10)
at ej.a(SourceFile:174)
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(UUIDTypeAdapter.java:22)
at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdapter.java:13)
at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdapter.java:10)
at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.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.YggdrasilAuthenticationService.makeRequest(YggdrasilAuthenticationService.java:54)
at com.mojang.authlib.yggdrasil.YggdrasilMinecraftSessionService.joinServer(YggdrasilMinecraftSessionService.java:61)
at bll.a(SourceFile:48)
at jt.a(SourceFile:41)
at jt.a(SourceFile:10)
at ej.a(SourceFile:174)
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: 1.7.9
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_51, Oracle Corporation
Java VM Version: Java HotSpot™ Client VM (mixed mode, sharing), Oracle Corporation
Memory: 228067616 bytes (217 MB) / 377180160 bytes (359 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_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: 13, tallocated: 95
Launched Version: 1.7.9
LWJGL: 2.9.1
OpenGL: GeForce GT 610/PCIe/SSE2/3DNOW! GL version 4.4.0, 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: [[1.6.X-1.7.X]_MegaDoku_TSC_Light.zip]
Current Language: English (US)
Profiler Position: N/A (disabled)
Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Anisotropic Filtering: Off (1)