title: GetAssetPreview always returns null
body: <p>I have a small unity editor script that gets asset previews from unity and saves them to a folder. It works well for my machine but it doesn't work on another machine that has the same project. 
  Here is the code I use:</p>
  
  <pre><code>GameObject[] objs = Resources.LoadAll&lt;GameObject&gt;("");
  foreach (GameObject obj in objs)
  {
      Texture2D previewImage = null;
      while (previewImage == null)
      {
          previewImage = AssetPreview.GetAssetPreview(obj);
          Thread.Sleep(100);
      }
      System.IO.File.WriteAllBytes(previewImage.name + ".png", previewImage.EncodeToPNG());
  }
  </code></pre>
  
  <p>The code stays in an infinite loop where it says <code>previewImage == null</code>. Any ideas ?  </p>