Null reference exception was unhandled?

Quote:

Bitmap histogram = new Bitmap(256, 256, PixelFormat.Format24bppRgb);

unsafe
        public static Bitmap CreateHistogram(Bitmap source, bool isGray)
        {
             if (source.PixelFormat == PixelFormat.Format24bppRgb)
                {
                    Bitmap histogram = new Bitmap(256, 256, PixelFormat.Format24bppRgb);
                    BitmapData data = source.LockBits(new Rectangle(0, 0, source.Width, source.Height),
                        ImageLockMode.ReadWrite, source.PixelFormat);

                    byte* p = (byte*)data.Scan0;
                    int offset = data.Stride - source.Width * 3;

                    if (isGray == false)
                    {
                        for (int hang = 0; hang < source.Height; hang++)
                        {
                            for (int cot = 0; cot < source.Width; cot++)
                            {
                                //0.21 R + 0.72 G + 0.07 B
                                byte t = (byte)(0.07f * p[0] + 0.72f * p[1] + 0.21 * p[2]);
                                p[0] = p[1] = p[2] = t;
                                p += 3;
                            }
                            p += offset;
                        }
                        p = (byte*)data.Scan0;
                    }



                    int[] count = new int[256];
                    int max = 0;
                    for (int hang = 0; hang < source.Height; hang++)
                    {
                        for (int cot = 0; cot < source.Width; cot++)
                        {
                            count[p[0]]++;


                            if (count[p[0]] > max)
                                max = count[p[0]];

                            p += 3;
                        }
                        p += offset;
                    }
                    source.UnlockBits(data);

                    // max 255
                    // x => x*255/max
                    for (int i = 0; i < 256; i++)
                        count[i] = (int)(count[i] * (histogram.Height - 1) * 1f / max * 1f);


                    data = histogram.LockBits(new Rectangle(0, 0, histogram.Width, histogram.Height),
                        ImageLockMode.ReadWrite, histogram.PixelFormat);
                    p = (byte*)data.Scan0;
                    offset = data.Stride - histogram.Width * 3;

                    for (int cot = 0; cot < histogram.Width; cot++)
                    {
                        for (int hang = 0; hang < histogram.Height; hang++)
                        {
                            byte value = 255;

                            if (hang <= (histogram.Height - count[cot]))
                                value = 255;
                            else
                                value = 0;

                            p[indexOf(hang, cot, data.Stride)] = value;
                            p[indexOf(hang, cot, data.Stride) + 1] = value;
                            p[indexOf(hang, cot, data.Stride) + 2] = value;

                        }
                    }
                    histogram.UnlockBits(data);
                    return histogram;
                }
                else
                {
                    MessageBox.Show("PixelFormat: " + source.PixelFormat);
                    return source;
                }

            }