How to work with custom class (ScriptableObject) SerializedProperty?

Hello, im trying to make customclasses that are derived from ScriptableObject work with SerializedProperty

Below, i have made a very minimal and simple example that works without deriving from ScriptableObject.

Human.cs

 public class Human : MonoBehaviour
 {
     public Weapon Weapon;
     public int Health;
 }

Weapon.cs

 [System.Serializable]
 public class Weapon
 {
     public int Damage;
     public int Weight;
 }

HumanEditor.cs

 [CustomEditor(typeof(Human))]
 public class HumanEditor : Editor
 {
     SerializedObject sObj;
 
     void OnEnable()
     {
         this.sObj = new SerializedObject(target);
 
         Weapon weapon = ((Human)target).Weapon;
         if (weapon == null)
             weapon = new Weapon();
     }
     public override void OnInspectorGUI()
     {
         this.sObj.Update();
 
         SerializedProperty sHealth = sObj.FindProperty("Health");
         sHealth.intValue = EditorGUILayout.IntField("Health", sHealth.intValue);
 
         SerializedProperty sWeaponDamage = sObj.FindProperty("Weapon.Damage");
         sWeaponDamage.intValue = EditorGUILayout.IntField("Damage", sWeaponDamage.intValue);
 
         SerializedProperty sWeaponWeight = sObj.FindProperty("Weapon.Weight");
         sWeaponWeight.intValue = EditorGUILayout.IntField("Weight", sWeaponWeight.intValue);
 
         this.sObj.ApplyModifiedProperties();
     }
 }

This works. But when i derived weapon from ScriptableObject, and change the OnEnable() function to:

     void OnEnable()
     {
         this.sObj = new SerializedObject(target);
         SerializedProperty sWeapon = sObj.FindProperty("Weapon");
 
         if (sWeapon.objectReferenceValue == null)
             sWeapon.objectReferenceValue = ScriptableObject.CreateInstance<Weapon>();
     }

Then i get an exepction saying its null. What should i do to make it work with ScriptableObject? The reason i need this, is because i want to use polymorphism and inheritance with Weapon.