Main Class(RealmOfTheDragons.java)

    @EventHandler
    public void Initialization(FMLInitializationEvent event) {
        proxy.Initialization(event); 
        proxy.render();
        GameRegistry.registerWorldGenerator(new RealmOfTheDragonsWorldGenerator(), 0);
        NetworkRegistry.INSTANCE.registerGuiHandler(this, new GuiHandler());
       
    }

 

ServerProxy

	public void render() {}

 

package com.TheRPGAdventurer.ROTD.client.initialization;

import org.lwjgl.input.Keyboard;

import net.minecraft.client.settings.KeyBinding;
import net.minecraftforge.fml.client.registry.ClientRegistry;
import net.minecraftforge.fml.relauncher.Side;
import net.minecraftforge.fml.relauncher.SideOnly;

public class ModKeys {
	
    public static final String KEY_CATEGORY = "key.categories.gameplay";
    public static  KeyBinding KEY_BREATH;
    
    public static void init() {
    	KEY_BREATH = new KeyBinding("key.dragon.breath", Keyboard.KEY_R, KEY_CATEGORY);
        ClientRegistry.registerKeyBinding(KEY_BREATH);
    }
    
}

ClientProxy

    @SideOnly(Side.CLIENT)
	@Override
	public void render() {ModKeys.init();}

 

 

CRASH REPORT: 

https://pastebin.com/b953r3Ja

You're trying to load a class that contains CLIENT only code on SERVER which is why you're getting this crash. I think annotating your ModKeys class with SideOnly should fix it but make sure to not access that class from anywhere else except your ClientProxy