[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

The documentation example for WWW.LoadFromCacheOrDownload() shows the code waiting for the download to finish before it does anything with the WWW. I assume trying to unload assets that haven’t even loaded yet is the problem.