private void RadPropertyGrid_EditorRequired(object sender, PropertyGridEditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(PropertyGridSpinEditor))
    {
        e.Editor = new MyPropertyGridSpinEditor();
    }
}

public class MyPropertyGridSpinEditor : PropertyGridSpinEditor
{
    public override object Value
    {
        get { return base.Value; }
        set 
        {
            RadSpinElement spinEditor = (RadSpinElement)this.EditorElement;
            if (value == null)
            {
                spinEditor.EnableNullValueInput = true;
            }
            else 
            {
                spinEditor.EnableNullValueInput = false;
            }

            base.Value = value;
        }
    }
}