How do I use PrefabUtility.InstantiatePrefab() ?

I'm extending the Transform editor with a button that clones game objects in the direction that I want.

For GOs that don't have a prefab connection, it simply uses Instantiate, and it works great, including setting the clone's transform parent as the same as the original GO.

If they have a prefab connection, I want the clone to preserve it, which is why I'm also using PrefabUtility.InstantiatePrefab. Unlike Instantiate, it doesn't take parameters to set the position.

https://docs.unity3d.com/ScriptReference/PrefabUtility.InstantiatePrefab.html

Here's the relevant piece of the code:

 public void CloneGO(string dir)
     {
         int i = 0;
         float originalPosX = obj.transform.position.x;
         float originalPosY = obj.transform.position.y;
 
         while (i < clones)
         {
             i++;
 
             if (dir == "up")
             {
                 newPos = new Vector2(originalPosX, originalPosY + (i * 0.4f));
             }
 
             // Other code here
 
             bool isPrefab = PrefabUtility.IsPartOfPrefabInstance(obj);
             // For Reddit: obj is Selection.activeGameObject;
 
             if (isPrefab)
             {
                 Debug.Log("Cloned a prefab");
                 cloneObj = PrefabUtility.InstantiatePrefab(obj) as GameObject;
                 cloneObj.transform.position = newPos;
             }
 
             else
             {
                 Debug.Log("Cloned a regular GO");
                 cloneObj = Instantiate(obj, newPos, Quaternion.identity);
             }
 
             cloneObj.transform.parent = obj.transform.parent;
 
         }

I'm getting a NullReferenceException when I try to clone a prefab. I also know that clone.Obj returns null in the line cloneObj = PrefabUtility.InstantiatePrefab(obj) as GameObject; but I don't understand why.