Need to correct this calculator program

Question

Unable to get the second value (var b) for calculation.

In this calculator program I got the first value (var a) from display.text and cleared the display.text.Then getting the second value (var b) for adding/Subtrating, the second value still assigned to var a and getting error

unexpectedly found nil for value b

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var display: UITextField!
    @IBOutlet weak var nine: UIButton!
    @IBOutlet weak var eight: UIButton!
    @IBOutlet weak var seven: UIButton!
    @IBOutlet weak var six: UIButton!
    @IBOutlet weak var five: UIButton!
    @IBOutlet weak var four: UIButton!

    @IBOutlet weak var clear: UIButton!
    @IBOutlet weak var result: UIButton!
    @IBOutlet weak var add: UIButton!
    @IBOutlet weak var subtract: UIButton!
    @IBOutlet weak var multiply: UIButton!
    @IBOutlet weak var three: UIButton!
    @IBOutlet weak var two: UIButton!
    @IBOutlet weak var one: UIButton!
    var res : Int = 0
    var a : Int = 0
    var b : Int = 0
    var actionBtn : Int = 0
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    @IBAction func numbers(_ sender: Any) {

        display.text = display.text! + String((sender as AnyObject).tag)
        print("displtxt = ", display.text)
    }
       
    @IBAction func functionality(_ sender: Any) {
        
        actionBtn = (sender as AnyObject).tag
        a = Int(display.text!)!
        print("a = ", a)
        display.text = ""
    }
    
    @IBAction func clearDisplay(_ sender: Any) {
        display.text = ""
    }
    
    @IBAction func results(_ sender: Any) {
        b = Int(display.text!)!
        switch  actionBtn {
                case 10:
                    print("b = ", b)
                    res = a*b
                    print("res = ", res)
        
                case 11:
                    res = a-b
        
                case 12:
                    res = a+b
        
                default:
                    display.text = ""
        }
    }
}