[1.7.10] Never-called code crashing (packets).

It's not like I can't deal with it, just wondering if the code is 100% not called somewhere where I don't want it to.

So this part:

instance.registerMessage(PacketOpenIntroductionGui.class, PacketOpenIntroductionGui.class, 100, Side.CLIENT);

Specifically defines (concluded from documentation) that RECEIVER IS CLIENT.

Why does this:

public class PacketOpenIntroductionGui implements IMessage, IMessageHandler<PacketOpenIntroductionGui, IMessage>
{
...whole this "needed" stuff.

@Override
public IMessage onMessage(PacketOpenIntroductionGui message, MessageContext ctx)
{
	System.out.println("RECEIVED");
	Minecraft.getMinecraft().displayGuiScreen(new GuiIntroduction());
	return null;
}
}

Crash when I run dedicated server pointing out NoClassDef? (There is no Minecraft on sever, i know that)

Note: On dedic there is NO print "RECEIVED" so I assumed it's not called.

What am I missing?

EDIT:

Turns out I can't deal with it:

Side side = FMLCommonHandler.instance().getEffectiveSide();        
	if (side == Side.CLIENT)
	{
		System.out.println("INTRODUCTION");
		Minecraft.getMinecraft().displayGuiScreen(new GuiIntroduction());
	}

Still gives me crash NoClassDef.