Cannot load an Asset Bundle - unity3d

Question

I created an asset bundle from an .fbx file. Then; I use this code for loading it into a very simple unity project
public class k : MonoBehaviour {
public string url;
// Use this for initialization
public string AssetName;
void Start()
{
}
IEnumerator DownloadAndCache(string bundleURL)
{
while (!Caching.ready)
yield return null;
using (WWW www = WWW.LoadFromCacheOrDownload(bundleURL, 1))
{
yield return www;
if (www.error != null)
throw new Exception("WWW download had an error:" + www.error);
AssetBundle bundle = www.assetBundle;
if (AssetName == "")
Instantiate(bundle.mainAsset);
else
Instantiate(bundle.LoadAsset(AssetName));
bundle.Unload(false);
}
}
// Update is called once per frame
void Update()
{
if (Input.GetKeyDown(KeyCode.D))
StartCoroutine(DownloadAndCache(url));
}
}
but always is having the next issue
and I don't understand. Since i'm passing a valid local url in the next format :
file:///c:/asset/robot.unity3d
please help

Answer 1

A file with a unity3d extension isn't usually an AssetBundle.
To have an AssetBundle with the model in it you have to:
Import the model in Unity
Select the imported model so you see its properties in the Inspector
At the bottom of the Inspector there is a little interface to create and select AssetBundles (this way the model will be assigned to the selected AssetBundle)
After assigning every asset to their AssetBundle, you need to build the Bundles
Put the bundles files on a server or in the StreamingAssets folder and use their URL
AssetBundle management in Unity is still a painfull process so I suggest you to thoroughly read the Guide to AssetBundles and Resources.
There is also an AssetBundleManager here but I find it's code is a mess.
It's buggy and almost impossible to decipher: I ended up writing my own so I can't really suggest you to use it yet (I linked it because it might get better in the future).

Answer 2

Try calling bundle.GetAllAssetNames() to see what is inside the bundle before trying to instantiate anything.
AssetBundle bundle = www.assetBundle;
foreach(var name in bundle.GetAllAssetNames()){
Debug.Log(name);
}
mainAsset may be null depending on how the AssetBundle was created, so maybe that is what happening here, does it gives you the null exception if you try loading assets by name?