如何在方法在鏈接列表中工作後添加？

Question

class ListNode 
{ 
    public object Data { get; private set; } 

    public ListNode Next { get; set; } 

    public ListNode(object Element) 
    { 
     Data = Element; 
    } 

    public ListNode(object Element, ListNode NextNode) 
    { 
     Data = Element; 
     Next = NextNode; 
    } 

    public ListNode() 
    { 

    } 
} 

class LinkedList 
{ 
    ListNode first; 
    ListNode last; 

    public LinkedList() 
    { 
     first = null; 
     last = null; 
    } 


    public ListNode Find(object After) 
    { 
     ListNode current = new ListNode(); 
     current= first; 
     while (current.Data != After) 
     current = current.Next; 
     return current; 
    } 

    public void Add(object newItem, object After) 
    { 
     ListNode current=new ListNode(); 
     ListNode newNode=new ListNode(); 
     current = Find(After); 
     newNode.Next = current.Next; 
     current.Next = newNode; 
    } 

    public void InsertAtFront(object Element) 
    { 
     if (IsEmpty()) 
     { 
      first = last = new ListNode(Element); 
     } 
     else 
     { 
      first = new ListNode(Element,first); 
     } 
    } 

    bool IsEmpty() 
    { 
     return first == null; 
    } 

    public void Display() 
    { 
     ListNode current = first; 
     while (current!=null) 
     { 
      Console.WriteLine(current.Data); 
      current = current.Next; 
     }   
    } 
} 

我實現Find方法後添加特定的元素，但當我調試它顯示我的對象引用未設置爲對象異常的實例。請在Find方法或Add After方法中指出我的錯誤。謝謝