How to set attribute to an XmlNode object which is not set to an instance or object. c#

Question

I cannot figure out how to set a default value to an XmlNode.

I have an XmlNode called RequirementMinTime and I want to set it to the value of "0" when that node is not in the xml document. Here is the code I'm trying which is not working.

        XmlReader reader = XmlReader.Create(xmlpath, settings);
        XmlDocument doc = new XmlDocument();

        doc.Load(reader);

       if (GlobalNode.SelectSingleNode("MinTimeMs") == null)
        {
            RequirementMinTime.Attributes["MinTimeMs"].Value = "0";
        }
        else
        {
            RequirementMinTime = GlobalNode.SelectSingleNode("MinTimeMs");
        }

I am getting the following error in the if statement

"System.NullReferenceException: 'Object reference not set to an instance of an object.'"

this is the object declaration:

    public static XmlNode RequirementMinTime
    {
        get;
        set;
    }

Answer 1

here is the solution

    XmlReader reader = XmlReader.Create(xmlpath, settings);
    XmlDocument doc = new XmlDocument();

    doc.Load(reader);

   if (GlobalNode.SelectSingleNode("MinTimeMs") == null)
    {
        XmlNode newNode = doc.CreateNode(XmlNodeType.Element, "MinTimeMs", "");
        newNode.InnerText = "0";
        GlobalNode.AppendChild(newNode);    
        RequirementMinTime = GlobalNode.SelectSingleNode("MinTimeMs");
    }
    else
    {
        RequirementMinTime = GlobalNode.SelectSingleNode("MinTimeMs");
    }