title: In-App Purchases: Crash when view disappear
body: <p>My application has a tabbar that contains 4 view controllers. The third view controller contains a "store in-app purchases". In this controller I use an object that manages in-app purchases (product request, purchase, transaction etc...) that allow me to get and show price description ecc.</p>
  
  <p>The problem is: If I change tabs while the request started the app crashes sometimes, but not always.</p>
  
  <p>I've to cancel the request in viewDidDisappear?
  [productsRequest cancel] this code crashes. </p>