title: View hierachy not prepared for constraint
body: <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
  
      self.blurView = UIVisualEffectView(effect: UIBlurEffect(style: .ExtraLight)) as UIVisualEffectView
      self.view.insertSubview(self.blurView, belowSubview: self.filterPanel)
  
      self.blurView.addConstraint(NSLayoutConstraint(item: self.blurView, attribute: NSLayoutAttribute.Top, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.Top, multiplier: 1, constant: 0))
      self.blurView.addConstraint(NSLayoutConstraint(item: self.blurView, attribute: NSLayoutAttribute.Bottom, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.Bottom, multiplier: 1, constant: 0))
      self.blurView.addConstraint(NSLayoutConstraint(item: self.blurView, attribute: NSLayoutAttribute.Leading, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.Leading, multiplier: 1, constant: 0))
      self.blurView.addConstraint(NSLayoutConstraint(item: self.blurView, attribute: NSLayoutAttribute.Trailing, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.Trailing, multiplier: 1, constant: 0))
  }
  </code></pre>
  
  <p>When the above code is called it crashes with the error "The view hierarchy is not prepared for the constraint". I can understand this error being generated if I'm trying to add the constraint before it's been added to the view but the view is inserted which is then followed by the constraints being added.</p>
  
  <p>Any ideas?</p>