.net c#尝试播放声音时给出Null参考异常

I am trying to insert a sound whenever there is a match in my game. However, I keep getting a NullReferenceException when I get the match. It goes to the line soundEffectInstance.Play();. Any help is greatly appreciated. I've attempted everything when trying to get it to work. Here is my code:

using System.Collections.Generic;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;

namespace AndyKlax.KlaxGame
{
internal class Grid
{
    private const int GridOrigin = 660;


    SoundEffect soundEffect;
    SoundEffectInstance soundEffectInstance;


    private readonly List<Block>[] _grid = new List<Block>[Klax.NumberOfColumns];

    public void LoadContent(ContentManager content)
    {

        soundEffect = content.Load<SoundEffect>(@"Sounds/24372^pop2.mp3");
        soundEffectInstance = soundEffect.CreateInstance();

    }

    internal Grid()
    {
        for (int i = 0; i < Klax.NumberOfColumns; i++ )
        {
            _grid[i] = new List<Block>();
        }
    }

    private bool RemoveBlocks()
    {
        var foundBlocksToRemove = false;

        //Mark all blocks in the grid as unmatched
        for (int column = 0; column < Klax.NumberOfColumns; column++)
        {
            for (int row =_grid[column].Count -1 ; row >=0 ; row--) //Loop backwards do its safe to remove
            {
                if (_grid[column][row].Delete)
                {
                    _grid[column].RemoveAt(row);
                    foundBlocksToRemove = true;
                }
            }
        }

        return foundBlocksToRemove;
    }



    private void SearchForMatches()
    {

        //Search the whole grid - look for matches in each direction and set the deleted flag for anything you find
        for (int column = 0; column < Klax.NumberOfColumns; column++)
        {
            for (int row = 0; row < _grid[column].Count; row++)
            {
                SearchForMatches(column, row, 1, 0);  //Horizontal
                SearchForMatches(column, row, 0, 1);  //Vertical
                SearchForMatches(column, row, 1, 1);  //Diagonal one way
                SearchForMatches(column, row, 1, -1);  //Diagonal the other way
            }
        }
    }

    private void SearchForMatches(int column, int row, int xStep, int yStep)
    {


        int startColorIndex = _grid[column][row].ColorIndex;
        int matchCount = 0;

        int x = column;
        int y = row;

        //Loop in the given direction till we run out of color or grid
        do
        {
            matchCount++;
            x += xStep;
            y += yStep;

        } while (x < Klax.NumberOfColumns && y < _grid[x].Count && y>0 && startColorIndex == _grid[x][y].ColorIndex);

        //If the match is long enough 
        if (matchCount >= 3)
        {
            //Then mark all those blocks for removal
            x = column;
            y = row;

            //Loop in the given direction till we run out of color or grid
            do
            {
                _grid[x][y].Delete = true;

                x += xStep;
                y += yStep;
                soundEffectInstance.Play();

            } while (x < Klax.NumberOfColumns && y < _grid[x].Count && startColorIndex == _grid[x][y].ColorIndex);
        }
    }

    private void ResetDeleteFlags()
    {
        //Mark all blocks in the grid as unmatched
        for (int column = 0; column < Klax.NumberOfColumns; column++)
        {
            for (int row = 0; row < _grid[column].Count; row++)
            {
                _grid[column][row].Delete = false;
            }
        }
    }

    internal void Draw(SpriteBatch spriteBatch)
    {
        //Like the paddle the position of the block is inferred by its position in the grid
        spriteBatch.Begin();
        for (int column = 0; column < Klax.NumberOfColumns; column++)
        {
            for (int row = 0; row < _grid[column].Count; row++)
            {
                _grid[column][row].Draw(spriteBatch, column, GridOrigin - row * Klax.Texture.Height);
            }
        }
        spriteBatch.End();
    }

    internal void Add(int column, Block block)
    {
        _grid[column].Add(block);

        //Search for matches
        do
        {
            ResetDeleteFlags();  
            SearchForMatches();  //Look for matches and mark their deleted flag
        } while (RemoveBlocks());  //If we removed any then iterate again as there may be new matches

    }

    public bool HasSpace(int column)
    {
        //Is there room in this column
        return _grid[column].Count < Klax.NumberOfRows;
    }
}

}

Model model = contentManager.Load<Model>( ".\\content\\models\\box" );