title: Swift 3 error: Type 'Any' has no subscript members
body: <p>So I know this question has been asked and answered numerous times before, but I just migrated my project to Swift 3 and Im getting a ton of these errors in my code that parses JSON and I couldn't quite find answers that made me understand how to resolve my specific issue. </p>
  
  <pre><code>guard let result = try JSONSerialization.jsonObject(with: data!, options: []) as? [String:AnyObject] else {
          return
      }
      guard let responseData = result["Data"] else { return }
      guard let userData = responseData["UserProfile"] else { return }
  
      var userProfileFieldsDict = [String: String]()
  
      if let sessionToken = userData!["CurrentSessionToken"] as? NSString {
          userProfileFieldsDict["sessionToken"] = String(sessionToken)
  
      }
  }
  </code></pre>
  
  <p>The <code>if let sessionToken</code> line throws the aforementioned error, but not quite sure how you're supposed to deal with this in Swift 3? Could someone explain and suggest a best practice fix? </p>
  
  <p>Thanks a bunch!</p>