title: SLComposeViewController for Twitter now crashing in iOS 8.3
body: <p>I have a <code>Tweetsheet</code> share option in my app. Just updated to iOS8.3 and now <code>SLComposeViewController</code> is throwing an error when I try to present the <code>Tweetsheet</code>: </p>
  
  <blockquote>
    <p>"Terminating app due to uncaught exception
    'NSInvalidArgumentException', reason: 'Application tried to present a
    nil modal view controller on target"</p>
  </blockquote>
  
  <pre><code>SLComposeViewController *tweetSheet = [SLComposeViewController composeViewControllerForServiceType:SLServiceTypeTwitter];
  [tweetSheet setInitialText:@"testing!"];
  [self presentViewController:tweetSheet animated:YES completion:nil];
  </code></pre>
  
  <p>I'm checking beforehand whether Twitter is available. Anyone else have this problem now?</p>