Math calculator crashed in some language/regional settings

The problem is not the backplane (I find those messages useless), but the Locale you use.

Where does it crash exactly ? Is it line 20 ?

import UIKit

class A_ViewController: UIViewController {

    @IBOutlet weak var originalMWTextField: UITextField!
    @IBOutlet weak var shutIDPPtextField: UITextField!
    @IBOutlet weak var trueVDtextField: UITextField!
    @IBOutlet weak var switchTapped: UISwitch!
    @IBOutlet weak var outputTextField: UITextField!
  
    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }
  
    @IBAction func calculateTappedButton(_ sender: Any) {

        let num = NumberFormatter()
        let firstValue = Double(num.number(from:originalMWTextField.text!)!)
        let secondValue = Double(num.number(from:shutIDPPtextField.text!)!)
        let thirdValue = Double(num.number(from:trueVDtextField.text!)!)
      
        if switchTapped.isOn {
            let outputValue = secondValue * 19.23/thirdValue + firstValue
          
            outputTextField.text = String(format:"%.2f ppg. Round up if required!", outputValue)
          
        } else {
            let outputValue = secondValue * 19.23/thirdValue * 3.281 + firstValue
          
             outputTextField.text = String(format:"%.2f ppg. Round up if required!", outputValue)
        }
        }
    
    @IBAction func gestureTapped(_ sender: Any) {
        view.endEditing(true)
    }
}

Clearly, you have locale as "en" ; when you type 1,2 (not 1.2) then you crash on line 20

You could have a test to check for comma or dot (it is not very clean though !)

let num = NumberFormatter()
print(num.locale!)     // to test locale on iPhone
var value = originalMWTextField.text!
if num.number(from:value) == nil {
    value = value.replacingOccurrences(of: ".", with: ",")
     if num.number(from:value) == nil {     // Test with comma
        value = "0"     // not a number at the end
     }
}


let firstValue = Double(truncating: num.number(from:value)!)
print(firstValue)

Yes Claude31, it was crashing in line 20 as you said.

Looks like the block of test code you proposed works just fine to prevent the failure and it still works in USA/UK regions as well. Thanks a lot for your help!