NullReference exception problem

Hi

I have the following code which produced the error "NullReferenceException: Object reference not set to an instance of an object" at the line where I try to enqueue one of the linkedDialogues but I'm not sure why I am getting the error because if I log the object I seem to be able to access it. The Dialogues extend ScriptableObject. I'm not sure if this is the problem. Any ideas anyone?

 float currentPosX = 20.0f;
         Queue<Dialogue> queue = new Queue<Dialogue>();
         queue.Enqueue(workingSequence.RootDialogue);
 
         while(queue.Count > 0)
         {
             Dialogue currentDialogue = queue.Dequeue();
             if(currentDialogue == null)
             {
                 Debug.Log("null");
                 continue;
             }
 
             for (int index = 0; index < currentDialogue.linkedDialogues.Count; index++ )
             {
                 if (currentDialogue.linkedDialogues[index] != null)
                 {
                     Debug.Log(currentDialogue.linkedDialogues[index].name);
                    //ERROR IN THIS LINE BUT CAN ACCESS OBJECT WITH ABOVE LOG STATEMENT 
                    queue.Enqueue(currentDialogue.linkedDialogues[index]);
                 }
             }
 
             nodes.Add(new DialogueNode(currentDialogue, this, nodes.Count + 1, new Rect(currentPosX, 20, 400, 0)));
             currentPosX += 200.0f;
         }