title: One of the subviews of a stack view not showing up at all
body: <p>I want my stack view to have three views: red image, blue image and register view. The thing is that while the red and blue button appear just fine, the register view does not.</p>
  
  <p><a href="https://i.stack.imgur.com/r0RD9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/r0RD9.png" alt="enter image description here"></a></p>
  
  <p>This is how I set up and place my stack view inside my view controller view:</p>
  
  <pre><code>func setupSocialStackView() {
      let socialStackView = UIStackView(arrangedSubviews: redImage, blueImage, registerView])
      socialStackView.axis = .vertical
      socialStackView.distribution = .fillProportionally
      view.addSubview(socialStackView)
      socialStackView.spacing = 8
  
      // NSLayoutAnchor constraints here to place stack view 
      // inside my view controller view
  }
  </code></pre>
  
  <p>This is the code for my register view which doesn't show up:</p>
  
  <pre><code>lazy var registerView: UIView = {
      let containerView = UIView()
  
      // Register button
      let registerButton = UIButton(type: .system)
      registerButton.setTitle("Register", for: .normal)
      registerButton.titleLabel?.font = UIFont.systemFont(ofSize: 12, weight: UIFontWeightLight)
      registerButton.titleLabel?.textAlignment = .center
      registerButton.setTitleColor(UIColor.black, for: .normal)
      registerButton.titleLabel?.textColor = UIColor(r: 91, g: 90, b: 90)
      registerButton.addTarget(self, action: #selector(presentRegisterController), for: .touchUpInside)
      containerView.addSubview(registerButton)
  
      return containerView
  }()
  </code></pre>
  
  <p>The other two arrange views of the stack view are <code>UIImageView</code>s.</p>
  
  <p>Why are the two images there and the register view is not? Am I missing something?</p>