title: Windows phone BackPressed event does not fire when I add PivotItem from code
body: <p>I develop app with setting page include Pivot. On NavigatedTo method, load data from db async and add PivotItem.</p>
  
  <p>BUT, BackPressed event does not fire for N times tap on back button (N = count of dynamic add PivotItem) and after it event call correctly. when i comment dynamic add PivotItem code, BackPressed event work properly.</p>
  
  <pre><code>    protected override void OnNavigatedTo(Windows.UI.Xaml.Navigation.NavigationEventArgs e)
      {            
          HardwareButtons.BackPressed += HardwareButtons_BackPressed;
  
              DataService.GetDataFromDB().ContinueWith(data =&gt;
              {
                  RunOnUiThread(() =&gt;
                  {
                      data.Result.ForEach(item =&gt;
                      {
                          var uc = new FolderUC(item);
                          var pivotItem = new PivotItem() { Header = item.Title, Content = uc };
                          pivot.Items.Add(pivotItem);
                      });
                  });
              });
      }
  </code></pre>
  
  <p>What is wrong????</p>