Hopper: java.lang.NoClassDefFoundError: io/netty/channel/EventLoopGroup

java.lang.NoClassDefFoundError: io/netty/channel/EventLoopGroup
	at bgb.<init>(SourceFile:33)
	at btj.a(SourceFile:268)
	at bcd.a(SourceFile:172)
	at btj.a(SourceFile:517)
	at bcd.d(SourceFile:236)
	at bcd.k(SourceFile:211)

---- Minecraft Crash Report ----
// Oops.

Time: 25.10.13 18:10
Description: Updating screen events

java.lang.NoClassDefFoundError: io/netty/channel/EventLoopGroup
	at bgb.<init>(SourceFile:33)
	at btj.a(SourceFile:268)
	at bcd.a(SourceFile:172)
	at btj.a(SourceFile:517)
	at bcd.d(SourceFile:236)
	at bcd.k(SourceFile:211)
	at azd.o(SourceFile:1303)
	at azd.ad(SourceFile:753)
	at azd.e(SourceFile:704)
	at net.minecraft.client.main.Main.main(SourceFile:103)
Caused by: java.lang.ClassNotFoundException: io.netty.channel.EventLoopGroup
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	... 10 more


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

-- Head --
Stacktrace:
	at bgb.<init>(SourceFile:33)
	at btj.a(SourceFile:268)
	at bcd.a(SourceFile:172)
	at btj.a(SourceFile:517)
	at bcd.d(SourceFile:236)
	at bcd.k(SourceFile:211)

-- Affected screen --
Details:
	Screen name: btj
Stacktrace:
	at azd.o(SourceFile:1303)
	at azd.ad(SourceFile:753)
	at azd.e(SourceFile:704)
	at net.minecraft.client.main.Main.main(SourceFile:103)

-- System Details --
Details:
	Minecraft Version: 1.7.2
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.6.0_37, Sun Microsystems Inc.
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Sun Microsystems Inc.
	Memory: 33332944 bytes (31 MB) / 70180864 bytes (66 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: 0, tallocated: 0
	Launched Version: 1.7.2
	LWJGL: 2.9.0
	OpenGL: AMD Radeon HD 6320 Graphics            GL version 4.1.10834 Compatibility Profile Context, ATI Technologies Inc.
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Packs: []
	Current Language: Deutsch (Deutschland)
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null
	Anisotropic Filtering: Off (1)