AssetPreview.GetAssetPreview(Selection.activeGameObject); is always null

I am trying to get the AssetPreview.GetAssetPreview(Selection.activeGameObject); texture but it is always returning null.

Does anyone know how I can get the AssetPreview.GetAssetPreview(Selection.activeGameObject);???

Code (CSharp):

1.     void OnGUI () {  
2.         int counter = 0;
3.         if(Selection.activeGameObject!=null)
4.             GUILayout.Label (Selection.activeGameObject.name+" is ready "+Selection.activeGameObject.GetInstanceID());
5.         else
6.             GUILayout.Label ("its null");
7.  
8.         while(AssetPreview.IsLoadingAssetPreviews() && counter<1000000)
9.             counter++;
10.        
11.         if(texThumb==null)
12.             texThumb=AssetPreview.GetAssetPreview(Selection.activeGameObject);
13.        
14.         if(texThumb!=null)
15.             GUILayout.Label(texThumb);
16.         else
17.             GUILayout.Label ("its still null ");          
18.     }
19.