그래프 강좌 실행시 Thread오류 납니다.

class ViewController: UIViewController {

@IBOutlet weak var graph1Height: NSLayoutConstraint!
@IBOutlet weak var graph2Height: NSLayoutConstraint!
@IBOutlet weak var graph3Height: NSLayoutConstraint!
@IBOutlet weak var graph4Height: NSLayoutConstraint!
@IBOutlet weak var graph5Height: NSLayoutConstraint!
override func viewDidLoad() {
    super.viewDidLoad()
    graph1Height = graph1Height.changeMultiplier(value: 0.9 as! CGFont) <- Thread 오류
    graph2Height = graph2Height.changeMultiplier(value: 0.8 as! CGFont)
    graph3Height = graph3Height.changeMultiplier(value: 0.7 as! CGFont)
    graph4Height = graph4Height.changeMultiplier(value: 0.8 as! CGFont)
    graph5Height = graph5Height.changeMultiplier(value: 0.5 as! CGFont)
}

}

extension NSLayoutConstraint {

func changeMultiplier(value: CGFont) -> NSLayoutConstraint {

    NSLayoutConstraint.deactivate([self])
    let newConstraint = NSLayoutConstraint.init(item: self.firstItem!, attribute: self.firstAttribute, relatedBy: self.relation, toItem: self.secondItem, attribute: self.secondAttribute, multiplier: value as! CGFloat, constant: self.constant)
    newConstraint.priority = self.priority
    newConstraint.shouldBeArchived = self.shouldBeArchived
    newConstraint.identifier = self.identifier
    NSLayoutConstraint.activate([newConstraint])
    return newConstraint
}

}