Can not find the error in the code (runtime error). Sprite variable remains null in the second burger class

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;

namespace Assignment4
{
    /// <summary>
    /// This is the main type for your game.
    /// </summary>
    public class Game1 : Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Burger burger;
        const int WindowWidth = 800;
        const int WindowHeight = 600;
        Texture2D sprite;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            burger = new Burger(Content, @"Graphics / burger", WindowWidth / 2, WindowHeight / 8, null);
        }

        /// <summary>
        /// Allows the game to perform any initialization it needs to before starting to run.
        /// This is where it can query for any required services and load any non-graphic
        /// related content.  Calling base.Initialize will enumerate through any components
        /// and initialize them as well.
        /// </summary>
        protected override void Initialize()
        {
            // TODO: Add your initialization logic here

            base.Initialize();
        }

        /// <summary>
        /// LoadContent will be called once per game and is the place to load
        /// all of your content.
        /// </summary>
        protected override void LoadContent()
        {
            // Create a new SpriteBatch, which can be used to draw textures.
            spriteBatch = new SpriteBatch(GraphicsDevice);

            // TODO: use this.Content to load your game content here

        }

        /// <summary>
        /// UnloadContent will be called once per game and is the place to unload
        /// game-specific content.
        /// </summary>
        protected override void UnloadContent()
        {
            // TODO: Unload any non ContentManager content here
        }

        /// <summary>
        /// Allows the game to run logic such as updating the world,
        /// checking for collisions, gathering input, and playing audio.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Update(GameTime gameTime)
        {

            // TODO: Add your update logic here

            base.Update(gameTime);
        }

        /// <summary>
        /// This is called when the game should draw itself.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            // TODO: Add your drawing code here
            spriteBatch.Begin();
            burger.Draw(spriteBatch);
            spriteBatch.End();

            base.Draw(gameTime);
        }
    }


  
    public class Burger
    {
        #region Fields

        // graphic and drawing info
        Texture2D sprite;
        Rectangle drawRectangle;

        // burger stats
        int health = 100;

        // shooting support
        bool canShoot = true;
        int elapsedCooldownMilliseconds = 0;

        // sound effect
        SoundEffect shootSound;

        #endregion

        #region Constructors

        /// <summary>
        ///  Constructs a burger
        /// </summary>
        /// <param name="contentManager">the content manager for loading content</param>
        /// <param name="spriteName">the sprite name</param>
        /// <param name="x">the x location of the center of the burger</param>
        /// <param name="y">the y location of the center of the burger</param>
        /// <param name="shootSound">the sound the burger plays when shooting</param>
        public Burger(ContentManager contentManager, string spriteName, int x, int y,
            SoundEffect shootSound)
        {
            LoadContent(contentManager, spriteName, x, y);
            this.shootSound = shootSound;
        }

        #endregion

        #region Properties

        /// <summary>
        /// Gets the collision rectangle for the burger
        /// </summary>
        public Rectangle CollisionRectangle
        {
            get { return drawRectangle; }
        }

        #endregion

        #region Public methods

        /// <summary>
        /// Updates the burger's location based on mouse. Also fires 
        /// french fries as appropriate
        /// </summary>
        /// <param name="gameTime">game time</param>
        /// <param name="mouse">the current state of the mouse</param>
        public void Update(GameTime gameTime, MouseState mouse)
        {
            // burger should only respond to input if it still has health

            // move burger using mouse


            // clamp burger in window



            // update shooting allowed
            // timer concept (for animations) introduced in Chapter 7

            // shoot if appropriate

        }

        /// <summary>
        /// Draws the burger
        /// </summary>
        /// <param name="spriteBatch">the sprite batch to use</param>
        public void Draw(SpriteBatch spriteBatch)
        {
            spriteBatch.Draw(sprite, drawRectangle, Color.CornflowerBlue);
        }

        #endregion

        #region Private methods

        /// <summary>
        /// Loads the content for the burger
        /// </summary>
        /// <param name="contentManager">the content manager to use</param>
        /// <param name="spriteName">the name of the sprite for the burger</param>
        /// <param name="x">the x location of the center of the burger</param>
        /// <param name="y">the y location of the center of the burger</param>
        private void LoadContent(ContentManager contentManager, string spriteName,
            int x, int y)
        {
            // load content and set remainder of draw rectangle
            sprite = contentManager.Load<Texture2D>(spriteName);
            drawRectangle = new Rectangle(x - sprite.Width / 2,
                y - sprite.Height / 2, sprite.Width,
                sprite.Height);
        }

        #endregion
    }
}