spoutlwc error

10:21:51 [SEVERE] Could not pass event CUSTOM_EVENT to SpoutLWC
java.lang.NoClassDefFoundError: com/griefcraft/util/StringUtils
        at tux2.spoutlwc.LWCScreenListener.createProtection(LWCScreenListener.java:199)
        at tux2.spoutlwc.LWCScreenListener.onButtonClick(LWCScreenListener.java:62)
        at org.getspout.spoutapi.event.screen.ScreenListener.onCustomEvent(ScreenListener.java:62)
        at org.bukkit.plugin.java.JavaPluginLoader$97.execute(JavaPluginLoader.java:945)
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:58)
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:339)
        at org.getspout.spoutapi.packet.PacketControlAction.run(PacketControlAction.java:113)
        at org.getspout.spout.packet.CustomPacket.a(CustomPacket.java:129)
        at net.minecraft.server.NetworkManager.b(NetworkManager.java:226)
        at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:93)
        at org.getspout.spout.SpoutNetServerHandler.a(SpoutNetServerHandler.java:586)
        at net.minecraft.server.NetworkListenThread.a(SourceFile:108)
        at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:527)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:425)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:457)
Caused by: java.lang.ClassNotFoundException: com.griefcraft.util.StringUtils
        at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:36)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:24)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
        ... 15 more