list.Add not accepting new class as instance?

Ok so I am trying to create a save load feature. Still. I am trying to save a list of classes that define the name, position, and rotation of the components. But for some reason when I run it in unity it throws up flags claiming that I am not using an instance of an object.

the function in question goes as follows:

 function AddNewPartData(newName : String, pos : Vector3, rot : Quaternion)
     {
         var NewPartData = new PartData();
         NewPartData.PartName = newName;
         NewPartData.PartPos = pos;
         NewPartData.PartRot = rot;
         ShipData.Add(NewPartData);
     }

As you can see I declare the NewPartData as a new PartData object then try to put it in the list. That's how you instance a non-monobehavior class right? Why is this not working?

Thanks in advance for any help provided

Full Code:

 import System.Collections.Generic;
 import System.Xml;
 import System.IO;
  
 @XmlRoot("ShipSave")
 public class ShipSave
 {
     @XmlArray("ShipData")
       @XmlArrayItem("Part")
     public var ShipData : List.<PartData>;
     
     function AddNewPartData(newName : String, pos : Vector3, rot : Quaternion)
     {
         var NewPartData = new PartData();
         NewPartData.PartName = newName;
         NewPartData.PartPos = pos;
         NewPartData.PartRot = rot;
         ShipData.Add(NewPartData);
     }
     
     function AddNewPartData(newName : String, pos : Vector3, rot : Quaternion, group : WeaponGroup)
     {
         var NewPartData = new PartData();
         NewPartData.PartName = newName;
         NewPartData.PartPos = pos;
         NewPartData.PartRot = rot;
         NewPartData.Group = group;
         ShipData.Add(NewPartData);
     }
     
     public function Save(path : String)
      {
          //TODO if file already exists, delete it first then write new one
          var serializer : XmlSerializer = new XmlSerializer(ShipSave);
          var stream : Stream = new FileStream(path, FileMode.Create);
          serializer.Serialize(stream, this);
          stream.Close();
      }
  
      public static function Load(path : String):ShipSave 
      {
          var serializer : XmlSerializer = new XmlSerializer(ShipSave);
          var stream : Stream = new FileStream(path, FileMode.Open);
          var result : ShipSave = serializer.Deserialize(stream) as ShipSave;
          stream.Close();
          return result;
      }
  
          //Loads the xml directly from the given string. Useful in combination with www.text.
          public static function LoadFromText(text : String):ShipSave
          {
          var serializer : XmlSerializer = new XmlSerializer(ShipSave);
          return serializer.Deserialize(new StringReader(text)) as ShipSave;
      }
 }
 
 public class PartData
 {
     var PartName : String;
     var PartPos : Vector3;
     var PartRot : Quaternion;
     var Group : WeaponGroup;
 }