Polymorphism and ScriptableObjects

Hello, I'm having an issue with passing scriptable objects to methods in another class, which is causing nullreferenceexceptions.

Fisrt, here is the structure of my classes:

I have a Item class deriving from ScriptableObject, and Weapon class that derives from Item, a weapon, VSPR which derives from the Weapon class, an Inventory Class that derives from ScriptableObject and a Player class.

Here is the code in my Inventory Class:

 public SBVPlayer owner;

 public List<SBVItem> itemInventory;

 public List<SBVWeapon> weaponInventory;

 

             

 public void AddItem(SBVItem item)

 {

     itemInventory.Add(item);

 }

 

 public void AddItem(SBVWeapon weap)

 {

     Debug.Log(weap);

     weaponInventory.Add(weap);

 }

From my player class, I am instantiating an Inventory and Weapon, which gets passed to the inventory's AddItem method like so:

         inventory =  ScriptableObject.CreateInstance("SBVInventory") as SBVInventory;

         inventory.AddItem(ScriptableObject.CreateInstance("VSPR") as VSPR);

So, this gives me the following error:

NullReferenceException: Object reference not set to an instance of an object SBVInventory.AddItem (.SBVWeapon weap) (at Assets/Scripts/Core/SBVInventory.cs:26) SBVPC.Start () (at Assets/Scripts/Core/SBVPC.cs:99)

However, the Debug.Log in the AddItem method works fine, and prints "VSPR"

I'm not sure if the problem is due to polymorphism or if I'm making a mistake. Any help would be appreciated. Thanks!