title: FBInterstitialAd showAdFromRootViewController failure
body: <p>I am developing an iOS app and have recently integrated the Facebook SDK for iOS.
  Currently using version 4.11.0 with <code>FBAdView</code> and <code>FBInterstitialAd</code> for Facebook Audience Network ads. The implementation in my app is almost identical to that in the AdUnitsSample app provided in the SDK. </p>
  
  <p>The <code>FBAdView</code> banner ads were fine, but I have found a serious problem with the <code>FBInterstitialAd</code>. The first time the interstitial is displayed, it works fine and dismisses correctly. However, if the user clicks through the ad (to display a webpage or App Store page in the in-app browser) and then uses the 'Cancel' or 'Done' buttons to return to the interstitial and then back to the app, it then fails to display the ad on subsequent requests. </p>
  
  <p>For the failed requests, the <code>loadAd</code> method works fine, and the <code>interstitialAdDidLoad:</code> delegate method is fired as usual, but when I call <code>showAdFromRootViewController:</code> nothing happens, apart from this warning in the console log:</p>
  
  <p><code>Warning: Attempt to present &lt;FBInterstitialAdInternal: 0x1626fc90&gt; on &lt;FBInterstitialAdInternal: 0x163488d0&gt; whose view is not in the window hierarchy!</code></p>
  
  <p>The only difference I can see between my app and the AdUnitsSample app is that my app uses a XIB based UI with <code>UINavigationController</code>, while the sample app uses Storyboard and a <code>UITabBarController</code>. The call I make to the <code>showAdFromRootViewController:</code> method sends my root view controller as the parameter, and this is the <code>UIViewController</code> which is currently on screen at the time.</p>
  
  <p>From the warning, it appears that the Facebook interstitial is trying to display itself on top of the previous interstitial (<code>FBInterstitialAd</code> is a <code>UIViewController</code> subclass) and I have no idea why. Unless I can solve this, I will just have to drop the interstitial ads, which is not great.</p>
  
  <p>Any ideas?</p>
  
  <p><strong>Update:</strong></p>
  
  <p>After playing around with the AdUnitsSample app some more, I realised another difference. The sample app uses ARC, but my app does not.
  Sure enough, when I turned ARC off in the sample app, I got exactly the same failure. I have no idea how to work around that one, though, and since my app depends upon some 3rd party libraries which aren't built for ARC, I cannot enable ARC.</p>