Unable to Json response is not working - ios

Question

Swift 3.0 (Xcode 8.3)
I'm trying to make a small program, that send a username to a web data base. I have found out how to send first_name, last_name,dob,owner_mobile,owner_email,
owner_password,choice_for_verification, by using in POST method. I obtain a JSON string earlier, then when I try to parse it, I get the above error on the try NSJSONSerialization line, on the as keyword. What did I do wrong? Thanks for your answers.
#IBAction func Login_Action(_ sender: Any)
{
var responseString : String!
var request = URLRequest(url: URL(string: "http://dev.justpick2go.com/cpanel/api/owner/ownerregistration.php")!)
request.setValue("Application/x-www.ownerregistration.php.com", forHTTPHeaderField: "Content-Type")
request.httpMethod = "POST"
let postString = "first_name=\(txtFirstName.text!)&last_name=\(txtLastName.text!)&dob=\(txtDOB.text!)&owner_mobile=\(txtMobileNo.text!)&owner_email=\(txtEmailID.text!)&owner_password=\(txtPassword.text!)" //&choice_for_verification=\(email)" // sending a parameters
print("\(postString)")
request.httpBody = postString.data(using: .utf8)
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
guard let data = data, error != nil else { //checking for fundamental error
print("Error is =\(String(describing: error))")
return
}
if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200
{ // checking for http errors
print("statusCode should be 200 , but is\(httpStatus.statusCode)")
print("response is =\(String(describing: response))")
}
responseString = String(data: data, encoding: .utf8)
print("ResponseString=\(responseString!)")
do {
let json : NSDictionary! = try! JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! NSDictionary
self.parseTheJSonData(JsonData: json)
}
catch
{
print(error)
}
}
task.resume()
}
func parseTheJSonData(JsonData : NSDictionary)
{
var successMessage : String = String()
var sampleCode : Int = Int()
let verificationAlert = UIAlertController()
if ((JsonData.value(forKey: "success") as! Int) == 1)
{
successMessage = "Login is Successful"
sampleCode = JsonData.value(forKey: "success") as! Int
verificationAlert.addAction(UIAlertAction(title: "No", style: .cancel, handler: nil))
verificationAlert.addAction(UIAlertAction(title: "Yes", style: .default, handler: { (Relogin) in
let Log = self.storyboard?.instantiateViewController(withIdentifier: "") as! LoginViewController
self.navigationController?.pushViewController(Log, animated: true)
self.present(Log, animated: true, completion: nil)
}))
}
else if ((JsonData.value(forKey: "success") as! Int) == 0)
{
sampleCode = JsonData.value(forKey: "success") as! Int
successMessage = "Please try again"
verificationAlert.addAction(UIAlertAction(title: "OK", style: .cancel, handler: nil))
}
verificationAlert.title = successMessage
OperationQueue.main.addOperation
{
self.present(verificationAlert, animated: true, completion: nil)
}
}