title: UI disappear after gradient background
body: <p>when I add a gradient color on my view, all of my UI disappear. </p>
  
  <p>I don't know why because it's just the background that I change.</p>
  
  <pre><code>func createGradientLayer() {
          let gradientLayer = CAGradientLayer()
  
          gradientLayer.frame = self.view.bounds
  
          gradientLayer.colors = [UIColor().HexToColor(hexString: "#783CBD", alpha: 1).cgColor, UIColor().HexToColor(hexString: "#BC1FFF", alpha: 1).cgColor]
          gradientLayer.startPoint = CGPoint(x: 0, y: 0)
          gradientLayer.endPoint = CGPoint(x: 1, y: 1)
          self.view.layer.addSublayer(gradientLayer)
      }
  </code></pre>