How to handle null in xml and c# -2

Let's consider that this was automatically generated xml file and in the second Address tag there is nothing. It is null.

If it was not null , we are going to make a list for address and then continue with a class declaration as below. But if we do so, because of a null address tag it gives us an error.

If possible I would like to add an if statement saying that

if it is null , store it as string. if it is not null , go with the list thing.

Could you please show me in code part how to do that?

Best regards

xml sample:

<?xml version="1.0"?>
<XMLCD>
 <Personnel>
  <AddressDirectory>
    <Owner>Jerry</Owner>
    <Age>29</Age>
    <Company>123</Company>
    <Address>
     <ST>
      <HouseNo>1</HouseNo>
      <StreetName>5th</StreetName>
      <Town>Elmsford</Town>
     </ST>
    </Address>
    <Address> 
    </Address>
  </AddressDirectory>
  <AddressDirectory>
      <Owner>Joe</Owner>
      <Age>24</Age>
      <Company>456</Company>
      <Address>
       <ST>
        <HouseNo>1</HouseNo>
        <StreetName>10Blvd</StreetName>
        <Town>StMichael</Town>
       </ST>
      </Address>
      <Address> 
      </Address>
  </AddressDirectory>
 </Personnel>
</XMLCD>

My code:

        OpenFileDialog ofd = new OpenFileDialog();
        opnFileName = ofd.FileName;

        XmlSerializer deserializer = new XmlSerializer(typeof(XMLCD));
        TextReader reader = new StreamReader(this.opnFileName);
        object obj = deserializer.Deserialize(reader);
        XMLCD XmlData = (XMLCD)obj;
        reader.Close();

    public class XMLCD
    {
         [XmlElement("Personnel")]
         public List<Personnel> PersonnelList = new List<Personnel>();

         public class Personnel
         {
           [XmlElement("AddressDirectory")]
           public List<AddressDirectory> AddressDirectoryList = new List<AddressDirectory>();

           public class AddressDirectory
           {
                 [XmlElement("Owner")]
                 public string Owner{ get; set; }

                 [XmlElement("Age")]
                 public string Age{ get; set; }

                 [XmlElement("Company")]
                 public string Company{ get; set; }


                 [XmlElement("Address")]
                 public List<Address> AddressList = new List<Address>();

                 public class Address
                 {
                       [XmlElement("ST")]
                       public List<ST> STList = new List<ST>();

                       public class ST
                       {
                         [XmlElement("HouseNo")]
                         public string HouseNo{ get; set; }

                         [XmlElement("StreetName")]
                         public string StreetName{ get; set; }

                         [XmlElement("Town")]
                         public string Town{ get; set; }
                }
     }
  }
}

The code above works great only for xml codes who have no null adress tags. When the xml code has null adress tags as shown above it gives an error.