I setup my SimpleNetworkWrapper in my preInit method of my base mode class. This works fine for dedicated servers and single player, but when my mods runs as a client in multiplayer, it crashes. How can I prevent my mod from setting up the SimpleNetworkWrapper when it is run as a client on multiplayer. I already tried

Minecraft.getMinecraft.isSinglePlayer()

to detect if my mod is running on an integrated server, but it always returns false.

I think you want to set up a proxy. Perhaps this thread could help

I fixed the issue for single player by making a global variable that the integrated server can set directly, nullifying the need for packets. Now my SimpleNetworkWrapper isn't working on multiplayer. I set it up in my server's preInit method with

net = NetworkRegistry.INSTANCE.newSimpleChannel("dataChannel");
net.registerMessage(PacketHandler.class, Packet.class, 0, Side.CLIENT);

and send it with

net.sendTo(new Packet(data), player);

I tested this code on single player without the global variable and it worked, but on multiplayer the client doesn't receive the packet.