I've done a similar message on a previous version, but this one crashes when I run it on the server. Specifically, when I do this line. Not sure why, since it works fine in this 1.11.2 version.

 

EDIT: Log:

---- Minecraft Crash Report ----
// Don't be sad, have a hug! <3

Time: 2017-07-04 08:58
Description: Exception in server tick loop

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from The Darkness (thedarkness)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/multiplayer/WorldClient
	at java.lang.Class.getDeclaredConstructors0(Native Method)
	at java.lang.Class.privateGetDeclaredConstructors(Class.java:2671)
	at java.lang.Class.getConstructor0(Class.java:3075)
	at java.lang.Class.newInstance(Class.java:412)
	at net.minecraftforge.fml.common.network.simpleimpl.SimpleNetworkWrapper.instantiate(SimpleNetworkWrapper.java:171)
	at net.minecraftforge.fml.common.network.simpleimpl.SimpleNetworkWrapper.registerMessage(SimpleNetworkWrapper.java:164)
	at thatmartinguy.thedarkness.TheDarkness.preInit(TheDarkness.java:48)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:630)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
	at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
	at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
	at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
	at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
	at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
	at com.google.common.eventbus.EventBus.post(EventBus.java:217)
	at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:252)
	at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:230)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
	at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
	at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
	at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
	at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
	at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
	at com.google.common.eventbus.EventBus.post(EventBus.java:217)
	at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:147)
	at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:604)
	at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:98)
	at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:335)
	at net.minecraft.server.dedicated.DedicatedServer.init(DedicatedServer.java:125)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:549)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.multiplayer.WorldClient
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 43 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@5486ee92 from coremod FMLCorePlugin
	at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:262)
	at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279)
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176)
	... 45 more
Caused by: java.lang.RuntimeException: Attempted to load class net/minecraft/client/multiplayer/WorldClient for invalid side SERVER
	at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:62)
	at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:258)
	... 47 more


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

-- System Details --
Details:
	Minecraft Version: 1.12
	Operating System: Windows 10 (amd64) version 10.0
	Java Version: 1.8.0_131, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 439347552 bytes (418 MB) / 566231040 bytes (540 MB) up to 1780482048 bytes (1698 MB)
	JVM Flags: 0 total; 
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	FML: MCP 9.40 Powered by Forge 14.21.1.2387 5 mods loaded, 5 mods active
	States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
	UCH	minecraft{1.12} [Minecraft] (minecraft.jar) 
	UCH	mcp{9.19} [Minecraft Coder Pack] (minecraft.jar) 
	UCH	FML{8.0.99.99} [Forge Mod Loader] (forgeSrc-1.12-14.21.1.2387.jar) 
	UCH	forge{14.21.1.2387} [Minecraft Forge] (forgeSrc-1.12-14.21.1.2387.jar) 
	UCE	thedarkness{0.0.1} [The Darkness] (TheDarkness) 
	Loaded coremods (and transformers): 
	Profiler Position: N/A (disabled)
	Is Modded: Definitely; Server brand changed to 'fml,forge'
	Type: Dedicated Server (map_server.txt)

 

Edited July 4, 20178 yr by That_Martin_Guy

You are referencing a client-only class in your common code, hence the crash.

The previous solution works because you are using a proxy.