I am trying to make a spawn egg that spawns the wither, I am using a custom ModSpawnEggItem class:
 

package com.plainplaying.test.items;

import io.netty.handler.codec.http2.Http2FrameLogger;
import net.minecraft.block.DispenserBlock;
import net.minecraft.dispenser.DefaultDispenseItemBehavior;
import net.minecraft.dispenser.IBlockSource;
import net.minecraft.entity.EntityType;
import net.minecraft.entity.SpawnReason;
import net.minecraft.item.ItemStack;
import net.minecraft.item.SpawnEggItem;
import net.minecraft.nbt.CompoundNBT;
import net.minecraft.util.Direction;
import net.minecraftforge.common.util.Lazy;
import net.minecraftforge.fml.RegistryObject;
import net.minecraftforge.fml.common.ObfuscationReflectionHelper;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class ModSpawnEggItem extends SpawnEggItem {
    protected static final List<ModSpawnEggItem> UNADDED_EGGS = new ArrayList<>();
    private final Lazy<? extends EntityType<?>> entityTypeSupplier;
    public ModSpawnEggItem(final RegistryObject<? extends EntityType<?>> entityTypeSupplier, int primaryColorIn, int secondaryColorIn, Properties builder) {
        super(null, primaryColorIn, secondaryColorIn, builder);
        this.entityTypeSupplier = Lazy.of(entityTypeSupplier::get);
        UNADDED_EGGS.add(this);
    }

    public static void initSpawnEggs(){
        final Map<EntityType<?>, SpawnEggItem> EGGS = ObfuscationReflectionHelper.getPrivateValue(SpawnEggItem.class, null, "field_195987_b");
        DefaultDispenseItemBehavior dispenseBehavior = new DefaultDispenseItemBehavior() {
            @Override
            protected ItemStack dispenseStack(IBlockSource source, ItemStack stack) {
                Direction direction = source.getBlockState().get(DispenserBlock.FACING);
                EntityType<?> type = ((SpawnEggItem) stack.getItem()).getType(stack.getTag());
                if (direction == Direction.WEST){
                    type.spawn(source.getWorld(), stack, null, source.getBlockPos().add(-1,0,0), SpawnReason.DISPENSER,direction!= Direction.UP, false);
                }
                else if (direction == Direction.EAST){
                    type.spawn(source.getWorld(), stack, null, source.getBlockPos().add(1,0,0), SpawnReason.DISPENSER,direction!= Direction.UP, false);
                }
                else if (direction == Direction.NORTH){
                    type.spawn(source.getWorld(), stack, null, source.getBlockPos().add(0,0,-1), SpawnReason.DISPENSER,direction!= Direction.UP, false);
                }
                else if (direction == Direction.SOUTH){
                    type.spawn(source.getWorld(), stack, null, source.getBlockPos().add(0,0,1), SpawnReason.DISPENSER,true, false);
                }
                else if (direction == Direction.DOWN){
                    type.spawn(source.getWorld(), stack, null, source.getBlockPos().add(0,-1,0), SpawnReason.DISPENSER,true, false);
                }else{
                    type.spawn(source.getWorld(), stack, null, source.getBlockPos().add(0,1,0), SpawnReason.DISPENSER,true, false);
                }
                stack.shrink(1);
                return stack;
            }
        };

        for (final SpawnEggItem spawnEgg : UNADDED_EGGS){
            EGGS.put(spawnEgg.getType(null), spawnEgg);
            DispenserBlock.registerDispenseBehavior(spawnEgg, dispenseBehavior);
        }
        UNADDED_EGGS.clear();
    }

    @Override
    public EntityType<?> getType(CompoundNBT nbt) {
        return this.entityTypeSupplier.get();
    }
}

I am creating the spawn egg like this:
 

public static final RegistryObject<SpawnEggItem> WITHER_SPAWN_EGG = ITEMS.register("wither_spawn_egg",
            () -> new ModSpawnEggItem(EntityType.WITHER, 0xF1BF7B, 0x705128,new Item.Properties().group(TestMod.TAB)));

and I get an error saying that I need an RegistryObject instead of EntityType, and I understand why, but I don't know how to fix. I want it to work with registry object, so i though maybe I can use .get() on the registry object, but I don't know how to make ModSpawnEggItem be compatible with EntityType, Can somone help me?

I understand the code, It's just that I don't understand how to make the code work

I personally would juggle the code so that it takes in an entity type instead as I don't think it's possible to access the wither's registry object as it were (I could be wrong though). If you want to have other instances of a class take in a registry object, you could always use .get() when calling the constructor or even have an overloaded constructer that takes in a registry object and calls .get() on it.

I am aware of ,get(), but i have absoulutely no clue on how to make the code work on EntityType instead of RegistryObject