unsafe image noise removal in c# (error : Bitmap region is already locked)

public unsafe Bitmap  MedianFilter(Bitmap Img)
{
    int Size =2;
    List<byte> R = new List<byte>();
    List<byte> G = new List<byte>();
    List<byte> B = new List<byte>();
    int ApetureMin = -(Size / 2);
    int ApetureMax = (Size / 2);
    BitmapData imageData = Img.LockBits(new Rectangle(0, 0, Img.Width, Img.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppRgb);
    byte* start = (byte*)imageData.Scan0.ToPointer ();
    for (int x = 0; x < imageData.Width; x++)
    {
        for (int y = 0; y < imageData.Height; y++)
        {
            for (int x1 = ApetureMin; x1 < ApetureMax; x1++)
            {
                int valx = x + x1;
                if (valx >= 0 && valx < imageData.Width)
                {
                    for (int y1 = ApetureMin; y1 < ApetureMax; y1++)
                    {
                        int valy = y + y1;
                        if (valy >= 0 && valy < imageData.Height)
                        {
                            // error come from here
                            Color tempColor = Img.GetPixel(valx, valy);

                            R.Add(tempColor.R);
                            G.Add(tempColor.G);
                            B.Add(tempColor.B);
                        }
                    }
                }
            }
        }
    }
    R.Sort();
    G.Sort();
    B.Sort();
    Img.UnlockBits(imageData);
    return Img;
}