title: FacebookSDK(4.7) Custom Login UI Button - Swift 2.0
body: <p>I am new to swift, How to resolve this error of optional <strong>"fatal error: unexpectedly found nil while unwrapping an Optional value"</strong>. This error occurs when i cancelled the Facebook login process.
  <a href="https://i.stack.imgur.com/K36YL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/K36YL.png" alt="error in code"></a></p>