[Asset Bundle] unable to download from server

I am trying to download asset bundle from dropbox and its showing error when it download the file it shows an error as - "NullReferenceException: Object reference not set to an instance of an object". And storing the object of AssetBundle in my own object.like this -

 IEnumerator LoadAsset () {
         
 using (WWW www = WWW.LoadFromCacheOrDownload
 ("https://www.dropbox.com/s/64geijvud5cuci5/MGame?dl=0",1)) {
 
     if (www.error != null)
         throw new UnityException ("WWW download had an error:" + www.error);
             
             if (www.assetBundle != null) 
             {
                  myAssetBundle = www.assetBundle.LoadAsset("myGame");
             }
             
         www.assetBundle.Unload (false);
     }
         yield return null;
 }

And when I click on error it shows error on "www.assetbundle.Unload(false)" this line.

This is how I am instantiating my object after downloading

 IEnumerator InstantiateLoadedAsset (Object objectName) {
         var g = Instantiate (objectName, Vector3.zero, Quaternion.identity) as GameObject;
         yield return null;
     }

can anybody tell me whats wrong with my code