Dispose WWW if timeout occurs in Unity3d

I am trying to dispose a WWW object if a timeout occurs. I am using following code:

Code (csharp):

1. WWW localWWW;
2.  
3. void Start ()
4. {
5.     stattTime = Time.time;
6.  
7.     nextChange = Time.time + rotationSpeed;
8.  
9.     StartCoroutine ("DownloadFile");
10.  
11. }
12.  
13. bool isStopped = false;
14. bool isDownloadStarted = false;
15. // Update is called once per frame
16. void Update ()
17. {   //2.0f as to simulate timeout
18.     if (Time.time > stattTime + 2.0f  !isStopped) {
19.         isStopped = true;
20.         isDownloadStarted = false;
21.         Debug.Log ("Downloading stopped");
22.         StopCoroutine ("DownloadFile");
23.         localWWW.Dispose ();
24.  
25.     }
26.     if (isDownloadStarted) {
27.  
28.     }
29.  
30.     if (Time.time > nextChange  isDownloadStarted) {
31.         Debug.Log ("Current Progress: " + localWWW.progress);
32.         nextChange = Time.time + rotationSpeed;
33.     }
34. }
35.  
36. IEnumerator DownloadFile ()
37. {
38.     isDownloadStarted = true;
39.     GetWWW ();
40.     Debug.Log ("Download started");
41.     yield return (localWWW==null?null:localWWW);
42.     Debug.Log ("Downlaod complete");
43.     if (localWWW != null) {
44.         if (string.IsNullOrEmpty (localWWW.error)) {
45.             Debug.Log (localWWW.data);
46.         }
47.     }
48. }
49.  
50. public void GetWWW ()
51. {
52.     localWWW = new WWW (@"http://www.sample.com");
53. }

But I am getting NullReferenceException: WWW class has already been disposed. TestScript+c__Iterator2.MoveNext () as exception.

I am not sure what I am doing wrong here.

Can anybody help me in this?

Thanks