[AssetBundle]object is not instantiate from my server

Hi

I have used the asset bunble for instantiate my asset in my scene. This is the first time i am started using the assetbundle. I droped the simple cube in my dropbox folder and i got the link. Then i have used this simple script to instantiate that cube.

 function Start () {
     var www = WWW ("https://www.dropbox.com/s/drvaucv7l61c77g/Cube.unity3d");
     yield www;
     // Get the designated main asset and instantiate it.
     Instantiate(www.assetBundle.mainAsset);
 }

but its not working it in my unity editor as well as the web applications. It showing the following error: NullReferenceException: Object reference not set to an instance of an object BundleLoader+c__Iterator0.MoveNext () (at Assets/Script/BundleLoader.cs:17)

How should i solve this. Please help me to get out of this.

Thanks.