title: applying constraints to facebook login button
body: <p>I'm trying to add constraints to facebook button to make it at the button, here is my code;</p>
  
  <pre><code>...
  let loginButton = FBSDKLoginButton()
  loginButton.readPermissions = ["public_profile", "email"]
  loginButton.center = self.view.center
  
  let myConstraint =
  NSLayoutConstraint(item: loginButton,
          attribute: NSLayoutAttribute.Bottom,
          relatedBy: NSLayoutRelation.Equal,
          toItem: self.view,
          attribute: NSLayoutAttribute.Bottom,
          multiplier: 1.0,
          constant: -20)
      self.view.addConstraint(myConstraint)
  
  loginButton.addConstraint(myConstraint)
  
  loginButton.delegate = self
  self.view.addSubview(loginButton)
  </code></pre>
  
  <p>it fails with error below;</p>
  
  <pre><code>The view hierarchy is not prepared for the constraint: &lt;NSLayoutConstraint:0x15fd258a0 FBSDKLoginButton:0x15fe562d0'Log in'.bottom == UIView:0x15fd2fa60.bottom - 20&gt;
  When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] to debug.
  </code></pre>
  
  <p>Any idea?</p>