setting the parent of a transform which resdes in a prefab is dusabled to prevent data corruption

I'm downloading the 3D model from server using of Assetbundle. I'm getting the model from url but when i attached to the gamer object of Prefab it showing error and the cube is not showing "setting the parent of a transform which resdes in a prefab is dusabled to prevent data corruption",

 public GameObject imageTarget;
 private GameObject mBundleInstance = null;
 
 using (WWW www = WWW .LoadFromCacheOrDownload(bundleURL, Version)) {
             yield return www;
             Debug.Log (Version);
             
             if (www .error != null)
                 throw new UnityException("WWW Download had an error: " + www .error);
             
             AssetBundle bundle = www .assetBundle;
             if (AssetName == "") {
                 Debug.Log ("null");
                 mBundleInstance = Instantiate (bundle.mainAsset) as GameObject;
 
                 
                     mBundleInstance.transform.parent = imageTarget.transform;
                     mBundleInstance.transform.localScale = new Vector3(0.3f, 0.3f, 0.3f);
                     mBundleInstance.transform.localPosition = new Vector3(0.0f, 0.15f, 0.0f);
                     mBundleInstance.transform.gameObject.SetActive(true);
                     
                     
 
             }