Animate background when entering foreground - ios

Question

I am working on a stopwatch app that begins flashing when approaching limit and again when overtime. Everything works great except for when the app is pushed to the background and then brought to the foreground again. At that time the animation will not start.
I have tried moving the code to applicationWillEnterForeground and applicationDidBecomeActive but it produces a fatal error:
unexpectedly found nil while unwrapping an Optional value
Am I missing something basic here?
There is more to the code of course (calculations, etc.), but this is relevant section. (Edit: I've expanded this to provide a better view of what I'm doing.)
#IBAction func start(_ sender: AnyObject) {
if timerOn == 0 {
myTimer = Timer.scheduledTimer(timeInterval: 0.1, target:self, selector: #selector(ViewController.updateCounter), userInfo: nil, repeats: true)
startTime = NSDate.timeIntervalSinceReferenceDate
timerOn = 1
redOrYellow = "white"
startButton.superview?.sendSubview(toBack: startButton)
background.superview?.sendSubview(toBack: background)
pickerMask.isHidden = false
}
}
....
func animateLight() {
if redOrYellow == "red" {
print("red")
//background.superview?.sendSubview(toBack: background)
UIView.animate(withDuration: 0.3, delay: 0.01, options:[UIViewAnimationOptions.repeat, UIViewAnimationOptions.allowUserInteraction], animations: {
self.background.backgroundColor = UIColor.red
self.background.backgroundColor = UIColor.white
}, completion: nil)
} else if redOrYellow == "yellow" {
print("yellow")
//background.superview?.sendSubview(toBack: background)
UIView.animate(withDuration: 1, delay: 0.1, options:[UIViewAnimationOptions.repeat, UIViewAnimationOptions.allowUserInteraction], animations: {
self.background.backgroundColor = UIColor.yellow
self.background.backgroundColor = UIColor.white
}, completion: nil)
}
}
func updateCounter() {
let currentTime = NSDate.timeIntervalSinceReferenceDate
var elapsedTime: TimeInterval = currentTime - startTime
totalTime = elapsedTime
if totalTime >= partBaseTimes[numberOfSelectedPart] {
if redOrYellow != "red" {
redOrYellow = "red"
animateLight()
}
} else if totalTime >= (partBaseTimes[numberOfSelectedPart] - 30) {
if redOrYellow != "yellow" {
redOrYellow = "yellow"
animateLight()
}
}
let minutes = UInt8(elapsedTime / 60.0)
elapsedTime -= (TimeInterval(minutes) * 60)
let seconds = UInt8(elapsedTime)
elapsedTime -= TimeInterval(seconds)
let fractionOfSecond = UInt8(elapsedTime * 10)
let strMinutes = String(format: "%02d", minutes)
let strSeconds = String(format: "%02d", seconds)
let strFractionOfSecond = String(format: "%01d", fractionOfSecond)
talkTimer.text = "\(strMinutes):\(strSeconds).\(strFractionOfSecond)"
}