[MC-1366] Minecraft crashes when you join a server you are already logged in on.

When you are logged in on a server and start a second Minecraft session and join the same server, the first client crashes with (for me) following description:

      *Minecraft has crashed!*      
      ----------------------      

Minecraft has stopped running because it encountered a problem; Unexpected error
This error has been saved to C:\Users\Meta\AppData\Roaming\.minecraft\crash-reports\crash-2012-10-30_20.45.01-client.txt for your convenience. Please include a copy of this file if you report this crash to anyone.



--- BEGIN ERROR REPORT 3af587c1 --------
Generated 30.10.12 20:45

- Minecraft Version: 1.4.2
- Operating System: Windows 7 (amd64) version 6.1
- Java Version: 1.7.0_09, Oracle Corporation
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
- Memory: 7923688680 bytes (7556 MB) / 8039104512 bytes (7666 MB) up to 8039104512 bytes (7666 MB)
- JVM Flags: 2 total; -Xms8000m -Xmx8000m
- AABB Pool Size: 10077 (564312 bytes; 0 MB) allocated, 1182 (66192 bytes; 0 MB) used
- LWJGL: 2.4.2
- OpenGL: GeForce GTX 570/PCIe/SSE2 GL version 4.2.0, NVIDIA Corporation
- Is Modded: Very likely
- Type: Client
- Texture Pack: Default
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: ~ERROR~ NullPointerException: null

java.lang.NullPointerException
	at aud.b(SourceFile:256)
	at net.minecraft.client.Minecraft.a(SourceFile:433)
	at awr.a(SourceFile:484)
	at db.a(SourceFile:27)
	at bx.b(SourceFile:341)
	at awr.d(SourceFile:98)
	at awz.b(SourceFile:61)
	at net.minecraft.client.Minecraft.l(SourceFile:1282)
	at net.minecraft.client.Minecraft.J(SourceFile:582)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT 1291c576 ----------

EDIT: THIS IS NOT A VANILLA ISSUE. I HAD OPTIFINE INSTALLED AND THAT SOMEHOW CAUSED THAT.