Open Radar

 
Summary:
The documentation for QLPreviewItem clearly states that previewItemURL should return nil until the item is available:

"If the item is not available for preview, this property’s getter method should return nil. In this case, the Quick Look preview controller displays a “loading” view."

However, doing so causes a crash.

Steps to Reproduce:
- Return nil from previewItemURL

See example project: http://cl.ly/1B0Z2o3K2V3J

Expected Results:
- The preview view controller will display a spinning loading indicator until some event that refreshes the controller. This is unclear in the documentation, but either calling reload on the controller, or preferably, sending a kvo notification for the previewItemURL key.

Actual Results:
The controller does show a loading screen, but only until the appearance animation completes, at which point the app crashes.