title: AVAssetWriter startSessionAtSourceTime: Cannot call method when status is 0
body: <p>Well I happily discovered that my app crash on the version downloaded from the Apple AppStore whereas it never did before. looking at the log I get the exception</p>
  
  <p><code>AVAssetWriter startSessionAtSourceTime: Cannot call method when status is 0</code></p>
  
  <p>Obviously this error does not occur in debug mode. But it never did in release mode before in a code that was shipped with another app and available on the appStore.</p>
  
  <p>Could it be the <code>directoryForTemporaryFiles</code> added in <code>iOS 8</code> the responsible for this status 0 ?</p>