Memory Leak and its solutions using weak declaration in Swift

I'm making a match-3 game using SpriteKit. Explanation: http://ift.tt/1wrZ5I3. Please refer to swm93's comment on page 4
This is a tutorial but it seems to have a memory leak in the code. Could anyone possibly download this swift project file and find what causes the memory leak and give possible solutions? The maker of this tutorial said that there is a memory leak in "handleSwipe(swap)" method and that we can fix it by adding "weak" to field declaration. I tried to write "weak var scene: GameScene?" but if I do, it says "scene is nil" even if I initialized it like this: "scene = GameScene(size: skView.bounds.size)" in "viewDidLoad()" function.
Below code is my "GameViewController.swift" and the rest of the classes can be downloaded through my link here.

import UIKit
import SpriteKit
import AVFoundation

class GameViewController: UIViewController {
    // The scene draws the tiles and cookie sprites, and handles swipes.
    var scene: GameScene!

    // The level contains the tiles, the cookies, and most of the gameplay logic.
    // Needs to be ! because it's not set in init() but in viewDidLoad().
    var level: Level!

    var movesLeft = 0
    var score = 0

    @IBOutlet weak var targetLabel: UILabel!
    @IBOutlet weak var movesLabel: UILabel!
    @IBOutlet weak var scoreLabel: UILabel!
    @IBOutlet weak var gameOverPanel: UIImageView!
    @IBOutlet weak var shuffleButton: UIButton!

    var tapGestureRecognizer: UITapGestureRecognizer!

    @IBAction func dismiss(sender: UIButton) {
        self.dismissViewControllerAnimated(true, completion: {})
    }

    lazy var backgroundMusic: AVAudioPlayer = {
        let url = NSBundle.mainBundle().URLForResource("Mining by Moonlight", withExtension: "mp3")
        let player = AVAudioPlayer(contentsOfURL: url, error: nil)
        player.numberOfLoops = -1
        return player
        }()

    override func prefersStatusBarHidden() -> Bool {
        return true
    }

    override func shouldAutorotate() -> Bool {
        return true
    }

    override func supportedInterfaceOrientations() -> Int {
        return Int(UIInterfaceOrientationMask.AllButUpsideDown.rawValue)
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        // Configure the view.
        let skView = view as! SKView
        skView.multipleTouchEnabled = false

        // Create and configure the scene.
        scene = GameScene(size: skView.bounds.size)
        scene.scaleMode = .AspectFill

        // Load the level.
        level = Level(filename: "Level_1")
        scene.level = level
        scene.addTiles()
        scene.swipeHandler = handleSwipe

        // Hide the game over panel from the screen.
        gameOverPanel.hidden = true
        shuffleButton.hidden = true

        // Present the scene.
        skView.presentScene(scene)

        // Load and start background music.
        backgroundMusic.play()

        // Let's start the game!
        beginGame()
    }

    func beginGame() {
        movesLeft = level.maximumMoves
        score = 0
        updateLabels()

        level.resetComboMultiplier()

        scene.animateBeginGame() {
            self.shuffleButton.hidden = false
        }

        shuffle()
    }

    func shuffle() {
        // Delete the old cookie sprites, but not the tiles.
        scene.removeAllCookieSprites()

        // Fill up the level with new cookies, and create sprites for them.
        let newCookies = level.shuffle()
        scene.addSpritesForCookies(newCookies)
    }

    // This is the swipe handler. MyScene invokes this function whenever it
    // detects that the player performs a swipe.
    func handleSwipe(swap: Swap) {
        // While cookies are being matched and new cookies fall down to fill up
        // the holes, we don't want the player to tap on anything.
        view.userInteractionEnabled = false

        if level.isPossibleSwap(swap) {
            level.performSwap(swap)
            scene.animateSwap(swap, completion: handleMatches)
        } else {
            scene.animateInvalidSwap(swap) {
                self.view.userInteractionEnabled = true
            }
        }
    }

    // This is the main loop that removes any matching cookies and fills up the
    // holes with new cookies. While this happens, the user cannot interact with
    // the app.
    func handleMatches() {
        // Detect if there are any matches left.
        let chains = level.removeMatches()

        // If there are no more matches, then the player gets to move again.
        if chains.count == 0 {
            beginNextTurn()
            return
        }

        // First, remove any matches...
        scene.animateMatchedCookies(chains) {

            // Add the new scores to the total.
            for chain in chains {
                self.score += chain.score
            }
            self.updateLabels()

            // ...then shift down any cookies that have a hole below them...
            let columns = self.level.fillHoles()
            self.scene.animateFallingCookies(columns) {

                // ...and finally, add new cookies at the top.
                let columns = self.level.topUpCookies()
                self.scene.animateNewCookies(columns) {

                    // Keep repeating this cycle until there are no more matches.
                    self.handleMatches()
                }
            }
        }
    }

    func beginNextTurn() {
       level.resetComboMultiplier()
        level.detectPossibleSwaps()
        view.userInteractionEnabled = true
        decrementMoves()
    }

    func updateLabels() {
        targetLabel.text = String(format: "%ld", level.targetScore)
        movesLabel.text = String(format: "%ld", movesLeft)
        scoreLabel.text = String(format: "%ld", score)
    }

    func decrementMoves() {
        --movesLeft
        updateLabels()

        if score >= level.targetScore {
            gameOverPanel.image = UIImage(named: "LevelComplete")
            showGameOver()
        }
        else if movesLeft == 0 {
            gameOverPanel.image = UIImage(named: "GameOver")
            showGameOver()
        }
    }

    func showGameOver() {
        gameOverPanel.hidden = false
        scene.userInteractionEnabled = false
        shuffleButton.hidden = true

        scene.animateGameOver() {
            self.tapGestureRecognizer = UITapGestureRecognizer(target: self, action: "hideGameOver")
            self.view.addGestureRecognizer(self.tapGestureRecognizer)
        }
    }

    func hideGameOver() {
        view.removeGestureRecognizer(tapGestureRecognizer)
        tapGestureRecognizer = nil

        gameOverPanel.hidden = true
        scene.userInteractionEnabled = true

        beginGame()
    }

    @IBAction func shuffleButtonPressed(AnyObject) {
        shuffle()

        // Pressing the shuffle button costs a move.
        decrementMoves()
    }
}