shouldPerformSegue.swift

override func shouldPerformSegue(withIdentifier identifier: String, sender: Any?) -> Bool {
        if ageTextField.text?.isEmpty == false,
           heightTextField.text?.isEmpty == false,
           weightTextField.text?.isEmpty == false,
           activityButtonText.titleLabel?.text?.isEmpty == false {
            return true
        } else {
            let controller = UIAlertController(title: "Please fill in all the blank", message: "You need to fill all the blank to calculate your BMR & TDEE", preferredStyle: .alert)
            let okAction = UIAlertAction(title: "OK", style: .default, handler: nil)
            controller.addAction(okAction)
            present(controller, animated: true, completion: nil)
            return false
            }
    }