Trying to understand why this crashes when switch changes - ios

Question

learning Swift 3 - not sure why the code to change the background color of my button crashes the app. myButton is a class variable and should be available to the function correct?
class ViewController: UIViewController {
var myButton : UIButton!
var mySwitch : UISwitch!
override func viewDidLoad() {
super.viewDidLoad()
let firstView = UIView()
firstView.backgroundColor = UIColor.white
view = firstView
let myNewView = UIView( frame: CGRect(x:65, y:100, width:300, height:200))
myNewView.backgroundColor = UIColor.blue
//myNewView.layer.cornerRadius = 25
//myNewView.layer.borderWidth = 2
//myNewView.layer.borderColor = UIColor.red.cgColor
self.view.addSubview(myNewView)
let switchDemo=UISwitch();
switchDemo.isOn = true
switchDemo.setOn(true, animated: false);
switchDemo.addTarget(self, action: #selector(ViewController.switchValueDidChange(sender:)), for: .valueChanged);
let myButton = UIButton(frame: CGRect(x: 0, y: 0, width: 144, height: 144))
myButton.backgroundColor = UIColor.red
myButton.addTarget(self, action: #selector(ViewController.ratingButtonTapped(sender:)), for: .touchUpInside)
self.view.addSubview(myButton)
myButton.addSubview(switchDemo);
}
func switchValueDidChange( sender:UISwitch){
print("Switch changed")
myButton.backgroundColor = UIColor.green
}
func ratingButtonTapped(sender:UIButton!)
{
print("Button It Works!!!")
}
}

Answer 1

First of all and strictly spoken myButton is an instance variable (actually a property) rather than a class variable.
let myButton = creates a local variable with the same name as the property but it is not the same object.
The solution is to omit let
myButton = UIButton(frame: CGRect(x: 0, y: 0, width: 144, height: 144))

Answer 2

It's simple. You declare a class property but you don't instantiate it. I your viewDidLoad you just instantiate a local variable myButton, instead of instantiating the class property.
Here is a fix:
myButton = UIButton(frame: CGRect(x: 0, y: 0, width: 144, height: 144))
Just remove the let keyword.

Answer 3

You did not instantiate myButton anywhere in your code, therefore it stays nil.
You are, however, declaring a local variable called myButton in viewDidLoad.
You should replace your instantiation with this:
myButton = UIButton(frame: CGRect(x: 0, y: 0, width: 144, height: 144))