My Code

Mod

package com.P4xil.p4xilmod;

import com.P4xil.p4xilmod.util.RegistryHandler;
import net.minecraft.item.ItemGroup;
import net.minecraft.item.ItemStack;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.fml.common.Mod;
import net.minecraftforge.fml.event.lifecycle.FMLClientSetupEvent;
import net.minecraftforge.fml.event.lifecycle.FMLCommonSetupEvent;
import net.minecraftforge.fml.javafmlmod.FMLJavaModLoadingContext;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;


@Mod("p4xilmod")
public class P4xilMod
{
    private static final Logger LOGGER = LogManager.getLogger();
    public static final String Mod_ID = "p4xilmod";

    public P4xilMod() {
        FMLJavaModLoadingContext.get().getModEventBus().addListener(this::setup);
        FMLJavaModLoadingContext.get().getModEventBus().addListener(this::doClientStuff);

        RegistryHandler.init();

        MinecraftForge.EVENT_BUS.register(this);
    }

    private void setup(final FMLCommonSetupEvent event){}

    private void doClientStuff(final FMLClientSetupEvent event) {

    public static final ItemGroup TAB = new ItemGroup("p4xilmodTab") {
     @Override
     public ItemStack createIcon() {
            return new ItemStack(RegistryHandler.RUBY.get());
        }
        };
    };
    }