title: No UIViewController supplied to the ad. Cannot continue
body: <p>I have a sprite-kit game, and the first ad loads on <code>gameOver</code>, but the following <code>gameOver</code>'s after that give the following error : </p>
  
  <pre><code>&lt;Google&gt; No UIViewController supplied to the ad. Cannot continue.
  </code></pre>
  
  <p>This is how my code looks:</p>
  
  <p>initWithSize:</p>
  
  <pre><code>-(id)initWithSize:(CGSize)size { 
      if (...]) {
          ...
  
          self.interstitial = [[GADInterstitial alloc] init];
          self.interstitial.adUnitID = @"ca-app-pub-9147160832662960/2548046734";
          GADRequest *request = [GADRequest request];
          // Requests test ads on simulators.
          [self.interstitial loadRequest:request];
          ...
      }
      return self;
  }
  </code></pre>
  
  <p>gameOver:</p>
  
  <pre><code>-(void)gameOver
  {
      self.isGameOver = YES;
      ...
  
      if ([self.interstitial isReady]) {
          self.interstitial.delegate = self;
          [self.interstitial presentFromRootViewController:_viewController];
      }
     ...
  }
  </code></pre>