How to work with LINQ in a NET CORE 3.1 driver

I am working in net core 3.1 with a database, and I want to obtain the products but with their respective category, subcategory and brand, in the database I only have the id obviously, that is, in the articles table I have idcategory idsubcategory and idbrand.

I have the following: IproductService interface:

using APIGESTION.model;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace APIGESTION.Services
{
    public interface IProductoService
    {
        public void Add(ProductoRequest model)
        {

        }
        List<Producto> GetProductos();
    }
}

And its implementation with LINQ:

using APIGESTION.model;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace APIGESTION.Services
{
    public class ProductoService : IProductoService
    {
        public void Add(ProductoRequest model)
        {

            using (gestionContext db = new gestionContext())
            {
                using (var transaction = db.Database.BeginTransaction())
                    try
                    {

                        var producto = new Producto();

                        producto.IdProducto = model.IdProducto;
                        producto.Precio = model.Precio;
                        producto.Stock = model.Stock;
                        producto.IdCategoria = model.idCategoria;
                        producto.nombreCategoria = model.nombreCategoria;
                        producto.idSubCategoria = model.idSubCategoria;
                        producto.nombreSubCategoria = model.nombreSubCategoria;
                        producto.idMarca = model.idMarca;
                        producto.nombreMarca = model.nombreMarca;
                        producto.CodAlfanumerico = model.CodAlfanumerico;
                        producto.CodBarra = model.CodBarra;
                        producto.Aliiva = model.Aliiva;
                        producto.Unidad = model.Unidad;
                        producto.Contenido = model.Contenido;
                        producto.costo = model.costo;
                      
                        db.Productos.Add(producto);
                        db.SaveChanges();

                   
                        transaction.Commit();

                    }
                    catch (Exception)
                    {
                        transaction.Rollback();
                        throw new Exception("Ocurrio un error en la insercion");
                    }
            }

        }

        public List<Producto> GetProductos()
        {
            using (gestionContext db = new gestionContext())
            {
              var productos=(from p in db.Productos join cat in db.Categoria on p.idCategoria equals cat.IdCategoria join
                subcat in db.Subcategoria on p.idSubCategoria equals subcat.IdSubcategoria join m in db.Marcas on
                p.idMarca equals m.IdMarca
                select new
                {
                    IdProducto = p.IdProducto,
                    Precio = p.Precio,
                    Stock = p.Stock,
                    IdCategoria = p.idCategoria,
                    IdSubcategoria = p.idSubCategoria,
                    IdMarca = p.idMarca,
                    CodAlfanumerico = p.CodAlfanumerico,
                    CodBarra = p.CodBarra,
                    Aliiva = p.Aliiva,
                    Unidad = p.Unidad,
                    Contenido = p.Contenido,
                    costo = p.costo,
                    NombreProducto = p.NombreProducto,
                    nombreCategoria = p.nombreCategoria,
                    nombreMarca = p.nombreMarca,
                });
                return (List<Producto>)productos;
            }

         
           
       
           
        }
    }
}

Now, how do I call the method that obtains the list in the item or product controller?

namespace APIGESTION.Controllers
{


    [Route("[controller]")]
    [ApiController]
    [Authorize]
    public class ProductoController : ControllerBase
    {
        private readonly gestionContext _context;
        private IProductoService _producto;
        public ProductoController(gestionContext context)
        {
            _context = context;
        }

        // GET: api/Cliente

        [HttpGet]
        public IActionResult GetProducto()
        {
           var Producto = _producto.GetProductos();
            return Producto;
        }
..demas codigo

I don't know if it's okay like that. It does not give a compilation error but when I go to my client application and request the data it tells me:

System.NullReferenceException: 'Object reference not set to an instance of an object.' _product was null.