How do I fix object reference not set to instance of object in my code.

namespace Semantics3API
{
    public class Offers
    {
        public string SellerName { get; set; }
        public string Price { get; set; }
        public string Currency { get; set; }
        public string Availability { get; set; }
    }
    //
    public class Stores
    {
        public string SkuInStore { get; set; }
        public string ProductUrlInStore { get; set; }
        public List<Offers> Offers { get; set; }
    }
    //
    public class Images
    {
        public string imageUrl;
    }
    //
    public class Product
    {
        public string Title { get; set; }
        public List<Images> Images { get; set; }
        public List<Stores> Stores { get; set; }
    }
}

And here is the error null reference

Stores store = new Stores();
store.SkuInStore = skuInStore;
store.ProductUrlInStore = productUrlInStore;
Offers offer = new Offers();
offer.SellerName = sellerName;
offer.Price = price;
offer.Currency = currency;
offer.Availability = availability;
//adding offer to store
store.Offers.Add(offer); // object reference not set to instance of object error :(