Game keep crashing

I have a problem about my game keep crashing because of my code.

Code:

package net.flexstudios.armorytests.items;

import com.google.common.collect.Multimap;
import net.minecraft.entity.ai.attributes.Attribute;
import net.minecraft.entity.ai.attributes.AttributeModifier;
import net.minecraft.entity.ai.attributes.Attributes;
import net.minecraft.inventory.EquipmentSlotType;
import net.minecraft.item.*;

public class PoweredChestplateItem extends ArmorItem {

    public PoweredChestplateItem(IArmorMaterial materialIn, EquipmentSlotType slot, Properties builder) {
        super(materialIn, slot, builder);
    }

    @Override
    public Multimap<Attribute, AttributeModifier> getAttributeModifiers(EquipmentSlotType equipmentSlot) {
        Multimap<Attribute, AttributeModifier> result = super.getAttributeModifiers(equipmentSlot);
        result.put(Attributes.ATTACK_DAMAGE, new AttributeModifier(ATTACK_DAMAGE_MODIFIER, "Armor dmg", (double)4, AttributeModifier.Operation.ADDITION));
        return result;
    }

}

A part of the crashing report:

Description: Initializing game

java.lang.UnsupportedOperationException: null
	at com.google.common.collect.ImmutableMultimap.put(ImmutableMultimap.java:427) ~[guava-21.0.jar:?] {}
	at net.flexstudios.armorytests.items.PoweredChestplateItem.func_111205_h(PoweredChestplateItem.java:19) ~[?:beta1.0.0] {re:classloading}
	at net.minecraftforge.common.extensions.IForgeItem.getAttributeModifiers(IForgeItem.java:77) ~[?:?] {re:classloading}
	at net.minecraft.item.ItemStack.func_111283_C(ItemStack.java:844) ~[?:?] {re:classloading}
	at net.minecraft.item.ItemStack.func_82840_a(ItemStack.java:614) ~[?:?] {re:classloading}
	at net.minecraft.client.Minecraft.lambda$populateSearchTreeManager$7(Minecraft.java:642) ~[?:?] 

Edited May 13, 20214 yr by ThreeShotMan33001
Code