I'm trying to save all offers of a villager on Items with a MerchantOffer property. I wrote a function to convert a MerchantOffers-object to and ItemStack[]:

public ItemStack[] generateOfferItems(MerchantOffers offers){
    ItemStack[] stacks = new ItemStack[offers.size()];
    for(int i = 0; i < offers.size(); i++){
        OfferItem offerItem = new OfferItem(offers.get(i));
        stacks[i] = new ItemStack(offerItem,1);
    }
    return stacks;
}

That's my OfferItem-class:

public class OfferItem extends Item {

    public MerchantOffer offer = null;

    public OfferItem() {
        super(new Item.Properties().tab(ModCreativeModeTab.TEST_TAB).stacksTo(1));
    }

    public OfferItem(MerchantOffer offer){
        super(new Item.Properties().tab(ModCreativeModeTab.TEST_TAB).stacksTo(1));
        this.offer = offer;
    }
}

But the problem is that in "OfferItem offerItem = new OfferItem(offers.get(i));" the game crashes with this error: "Registry is already frozen".

The exception was thrown in the "guardEntityTick"-function of the Level.java file.

You don't create items at runtime.

If you created a new Item on a server during the game, how would all the clients know what this item means?

 

ItemStacks hold the instance data of items.

You need one OfferItem and then add to the NBT of the ItemStack by creating your own tag. ItemStack.getOrCreateTag() -

Usually you use your modid as the top level element name to avoid collisions.

MerchantOffer already has createTag() method to get it in NBT form.

If you want examples, research how enchantments are stored in ItemStacks.