title: execute on swift3 error
body: <p>I'm doing a log in Swift 3 and I've found an error that is probably silly but I'm stuck and blocked. I need help if you're so kind. I attach the piece of code and also a photo to make everything clearer. Thank you</p>
  
  <p><img src="https://i.stack.imgur.com/zYV3w.png" alt="Screenshot of error"></p>
  
  <pre><code>        if let data_block = server_response["data"] as? NSDictionary
          {
              if let session_data = data_block["session"] as? String
              {
                  let preferences = UserDefaults.standard
                  preferences.set(session_data, forKey: "session")
  
                  DispatchQueue.main.async {
                      execute:self.LoginDone()
                  }
              }
          }
      })
      task.resume()
  }
  </code></pre>