what does Error "Thread 1:EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)" mean?

Question

I got this error:

Thread 1:EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

How can I solve this? The code works normally, but in the calculator when I click the only equal button, it gives that error.

@IBAction func equals(sender: AnyObject) {

    secondNumber = Screen.text!.toInt()!  // here it shows an error which is "Thread 1:EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)"

    if operation == "+"{
        result = firstNumber + secondNumber
    }
    else if operation == "-" {
        result = firstNumber - secondNumber
    }
    else if operation == "x" {
        result = firstNumber * secondNumber
    }
    else {
        result = firstNumber / secondNumber
    }
    Screen.text = "\(result)"
}

Answer 1

This line

secondNumber = Screen.text!.toInt()!

means: Get the Screen object, get the text property and please crash if it doesn't exist, then get the text converted to an integer, and please crash if it doesn't exist.

That's what the ! means: "I am sure this thing exists, so please crash if it doesn't". And crash is what it did.