Adding .removeStateDidListener to deinit causes Application to Crash

// Environment: Xcode 11.1, Swift 5.1, iOS 10.3
deinit {
    if #available(iOS 11.0, *) {} else if let observer = observer {
        removeObserver(observer, forKeyPath: "foo")
    }
} 

// Environment: Xcode 11.1, Swift 5.1, iOS 10.3
deinit {
    // Also tried only either one, no luck
    self.observer?.invalidate() 
    self.observer = nil
}

keyWindow.rootViewController = redVC

protocol Hello {
    func thisGetsCalledSometimes()
}

class MyService {

    var delegate: Hello?

    init(key: String) {
        debugPrint("Init")
    }

    func start() {
        debugPrint("Service Started")
    }

    func stop() {
        debugPrint("Service Stopped")
    }
}

class MyClass: Hello {

    lazy var service: MyService = {
        // To init and configure this service,
        // we need to reference `self`.
        let service = MyService(key: "") // Just pretend key exists :)
        service.delegate = self
        return service
    }()

    func thisGetsCalledSometimes() {
        // Calling this function causes the lazy var to
        // get initialised.
        self.service.start()
    }

    deinit {
        // If `thisGetsCalledSometimes` was NOT called,
        // this crashes because the initialising closure
        // for `service` references `self`.
        self.service.stop()
    }
}

"Init"
"Service Stopped"

import UIKit

// Stuff to create a view stack:

class ViewController: UINavigationController {
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        let firstController = FirstController()
        let navigationController = UINavigationController(rootViewController: firstController)
        self.present(navigationController, animated: false, completion: nil)
    }
}

class FirstController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let button = UIButton()
        button.setTitle("Next screen", for: .normal)
        button.addTarget(self, action: #selector(onNextScreen), for: .touchUpInside)
        self.view.addSubview(button)
        button.translatesAutoresizingMaskIntoConstraints = false
        button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
        button.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
    }

    @objc func onNextScreen() {
        let secondController = SecondController()
        self.navigationController?.pushViewController(secondController, animated: true)
    }
}

// The service and view controller where the crash happens:

protocol ServiceDelegate: class {
    func service(_ service: Service, didReceive value: Int)
}

class Service {
    weak var delegate: ServiceDelegate?

    func start() {
        print("Starting")
        self.delegate?.service(self, didReceive: 0)
    }

    func stop() {
        print("Stopping")
    }

    deinit {
        delegate = nil
    }
}

class SecondController: UIViewController {

    private var isServiceAvailable: Bool = false

    private lazy var service: Service = {
        let service = Service()
        service.delegate = self
        //Make the service available
        self.isServiceAvailable = true
        return service
    }()

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
//                service.start() // <- Comment/uncomment to toggle crash
    }

    deinit {
        if self.isServiceAvailable {
            self.service.stop()
        }
    }
}

extension SecondController: ServiceDelegate {
    func service(_ service: Service, didReceive value: Int) {
        print("Value: \(value)")
    }
}

class TestAudioClass {

    private var audioEngine: AVAudioEngine
    private var sampler: AVAudioUnitSampler
    private var sequencer: AVAudioSequencer!

    init() {
        self.audioEngine = AVAudioEngine()
        self.sampler = AVAudioUnitSampler()
        audioEngine.attach(sampler)
        audioEngine.connect(sampler, to: audioEngine.mainMixerNode, format: nil)
        self.sequencer = AVAudioSequencer(audioEngine: audioEngine)
        if let fileURL = Bundle.main.url(forResource: "TestMusic", withExtension: "mid") {
            do {
                try sequencer.load(from: fileURL, options: AVMusicSequenceLoadOptions())
            } catch {
                print("Error loading sequencer: \(error.localizedDescription)")
            }
        }
        sequencer.prepareToPlay()
    }

    deinit {
        sequencer.stop()
        sequencer = nil
    }
}