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iOS Apprentice v8.1 Chapter 32 Fatal Error

Howdy, in the above named book I am stuck at page872. I’ve entered the code ending with the lazy var managedObjectContext and just before the section 'Browsing the data" when I run my app it crashes with this error: “Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value” on this line: “let tabController = window!.rootViewController as ! UITabBarController”. I’ve looked this over for 2 days and can’t find the issue. When I run the code that comes with the book it works fine.

Any help would be appreciated.
Thanks,
Keith

@keith99 Do you still have issues with this?