Download assetBundle from server

Hi every one, im trying to donwload asset bundle and then instantiate any gameobject from that. But i dont understand what im doing wrong. i was watching some tutorials, but still doeanst work. Canu help me pls? When im strarting Scene console write me this error :

NullReferenceException: Object reference not set to an instance of an object loadAsset+c__Iterator0.MoveNext () (at Assets/Scripts/loadAsset.cs:16)

and here is my script :

 using UnityEngine;
 using System.Collections;
 
 public class loadAsset : MonoBehaviour {
 
     private string url = "https://www.dropbox.com/s/juk97y0ovyruhpc/skuska.unity3d";
         
 
     IEnumerator Start () {
         WWW www = WWW.LoadFromCacheOrDownload (url,1);
 
         yield return www;
 
         AssetBundle bundle = www.assetBundle;
 
         GameObject kocka = bundle.Load("Cube", typeof(GameObject)) as GameObject;
 
         Instantiate(kocka);
 
         kocka.transform.Translate (new Vector3(1,1,1));
     }
 
 }

i was trying to use other server but still doeasnt work. Can u help with my problem? oh sure script its name is "ExportAssetBundles" i have in Editor Folder in my project.

Thanks Thomas