I know there's alot of code here but.....

I wrote a column header class. When clicked, I want to raise an event. I'm getting the error "Object reference not set to an instance of an object." in the OnSortOrderChanged at the bottom:

public enum SortOrder
{
    Ascending,
    Descending,
    None
}

public delegate void SortOrderEventHandler(object sender, SortOrderEventArgs e);

public partial class ColumnHeader : UserControl
{
    public event SortOrderEventHandler SortOrderChanged;

    private const int SORT_ARROW_TOP = 5;
    private const int SORT_ARROW_MARGIN = 5;
    private const int SORT_ARROW_SIZE = 12;

    private SortOrder SortBy = SortOrder.None;

    private string _Caption = "label1";
    public string Caption
    {
        get { return _Caption; }
        set 
        { 
            _Caption = value;
            lblCaption.Text = _Caption;
            this.Invalidate();
        }
    }


    public ColumnHeader()
    {
        InitializeComponent();
    }


    private void _ColumnHeader_Paint(object sender, PaintEventArgs e)
    {
        // Pens and Brushes
        Pen penGrey = new Pen(System.Drawing.Color.Gray);
        Pen penDarkGray = new Pen(System.Drawing.Color.DarkGray);
        Pen penWhite = new Pen(System.Drawing.Color.White);
        SolidBrush myBrush = new SolidBrush(Color.DarkGray);

        // Verticle lines on right edge
        e.Graphics.DrawLine(penGrey, this.Width - 2, 2, this.Width - 2, this.Height - 2);
        e.Graphics.DrawLine(penWhite, this.Width - 1, 2, this.Width - 1, this.Height - 2);

        // Sort order indicator
        int iLeft = this.Width - SORT_ARROW_SIZE - 5;
        Point[] points = new Point[3];
        switch (SortBy)
        { 
            case SortOrder.Ascending:
                points[0] = new Point(iLeft, SORT_ARROW_TOP);
                points[1] = new Point(iLeft - 5, SORT_ARROW_SIZE);
                points[2] = new Point(iLeft + 5, SORT_ARROW_SIZE);
                break;

            case SortOrder.Descending:
                points[0] = new Point(iLeft - 5, SORT_ARROW_TOP);
                points[1] = new Point(iLeft + 5, SORT_ARROW_TOP);
                points[2] = new Point(iLeft, SORT_ARROW_SIZE);
                break;
        }

        if (SortBy != SortOrder.None)
        {
            e.Graphics.DrawPolygon(penDarkGray, points);
            e.Graphics.FillPolygon(myBrush, points);
        }

        penGrey.Dispose();
        penDarkGray.Dispose();
        penWhite.Dispose();
    }
    private void _ColumnHeader_Resize(object sender, EventArgs e)
    {
        this.Invalidate();
    }
    private void _ColumnHeader_Click(object sender, EventArgs e)
    {
        _SetSortOrder();
    }

    private void _SetSortOrder()
    {
        switch (SortBy)
        { 
            case SortOrder.None:
                SortBy = SortOrder.Descending;
                break;

            case SortOrder.Descending:
                SortBy = SortOrder.Ascending;
                break;

            case SortOrder.Ascending:
                SortBy = SortOrder.None;
                break;
        }
        this.Invalidate();

        OnSortOrderChanged(new SortOrderEventArgs(SortBy));
    }



    protected virtual void OnSortOrderChanged(SortOrderEventArgs e)
    {
        SortOrderChanged(this, e);
    }
}

public class SortOrderEventArgs : EventArgs
{
    private SortOrder _SortOrder = SortOrder.None;
    public SortOrder SortOrder
    {
        get { return _SortOrder; }
    }
    public SortOrderEventArgs(SortOrder NewOrder)
    { 
        _SortOrder = NewOrder;
    }

}