title: Getting error "Callable returned null" when using RxJava2
body: <p>I am using RxJava2 in my android project. </p>
  
  <p>I am using the following code to create the <code>Observable</code></p>
  
  <pre><code>public Observable&lt;AlbumDetails&gt; loadAlbumFromAlbumId(final String albumId) {
      return Observable.fromCallable(new Callable&lt;AlbumDetails&gt;() {
          @Override
          public AlbumDetails call() throws Exception {
               AlbumDetails albumDetails = getAlbumDetails(albumId);     
               return albumDetails;   
         });
  }
  </code></pre>
  
  <p>From the observable, I am getting following error in the onError method of DisposableObserver</p>
  
  <p><code>Callable returned null</code></p>
  
  <p>This didn't use to happend when using RxJava.</p>