sprite.X = getIntAttribute(reader, "x");

<Texture image="astro.png" width="221" height="611">
    <region n="astro_E.png" x="0" y="0" w="56" h="163"/>
    <region n="astro_ENE.png" x="180" y="0" w="41" h="151"/>
    <region n="astro_ESE.png" x="177" y="151" w="41" h="151"/>
    <region n="astro_N.png" x="56" y="0" w="62" h="149"/>
    ...
</Texture >

public class Texture
{
    public int Width    { get; set; }
    public int Height   { get; set; }
    public string ImagePath { get; set; }
    public List<Texture> SpriteList { get; set; } = new List<Texture>();
}

public class Region
{
    public int Height   { get; set; }
    public int Width    { get; set; }
    public int X { get; set; }
    public int Y { get; set; }
    public string Name { get; set; }
    public bool Rotate { get; set; }
}

public void read(string filename)
{

    Texture textureAtlas = new Texture();

    using (var reader = XmlReader.Create(filename))
    {
        reader.MoveToContent();
        textureAtlas.Height = getIntAttribute(reader, "height");
        textureAtlas.Width = getIntAttribute(reader, "width");
        textureAtlas.ImagePath = getAttribute(reader, "imagePath");

        while (reader.Read())
        {
            Region sprite = new Region();

            reader.MoveToContent();

            sprite.Name = getAttribute(reader, "n");
            sprite.X = getIntAttribute(reader, "x"); //ArgumentNullException here
            sprite.Y = getIntAttribute(reader, "y");
            sprite.Width = getIntAttribute(reader, "w");
            sprite.Height = getIntAttribute(reader, "h");
            sprite.Rotate = getBoolAttribute(reader, "r");

            textureAtlas.SpriteList.Add(sprite);
        }
    }
}

private string getAttribute(XmlReader reader, string attributeName)
{
    return reader.GetAttribute(attributeName);
}

private int getIntAttribute(XmlReader reader, string attributeName)
{
    return int.Parse(reader.GetAttribute(attributeName));
}