title: Change width constraint of a Button in StackView Programmatically
body: <p>I have a stackView which contains some UItextfields and one button.
  I have constrained it in ViewController viewDidLoad like this:</p>
  <pre><code>let stackView = UIStackView()
      stackView.axis = .vertical
      stackView.spacing = 40
      stackView.distribution = .fillEqually
      stackView.alignment = .fill
       
       stackView.addArrangedSubview(justAskYouSomeInfosLabel)
       stackView.addArrangedSubview(emailTextField)
       stackView.addArrangedSubview(passwordTextField)
       stackView.addArrangedSubview(againPasswordTextField)
       stackView.addArrangedSubview(registrationButton)
       
       
               //autolayout the stackview
       view.addSubview(stackView)
       stackView.anchor(top: view.safeAreaLayoutGuide.topAnchor, leading: nil, bottom: nil, trailing: nil, padding: .init(top: 80, left: 0, bottom: 0, right: 0))
       stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
       stackView.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.8).isActive = true
  </code></pre>
  <p>Now I want to reduce the width of the button on the bottom of the stackView.</p>
  <p>I tried this:</p>
  <pre><code>registrationButton.widthAnchor.constraint(equalToConstant: 80).isActive = true
  </code></pre>
  <p>as suggested on other stackOverflow questions but it doesn't work and I receive this error on the console:</p>
  <pre><code>Unable to simultaneously satisfy constraints.
      Probably at least one of the constraints in the following list is one you don't want. 
  </code></pre>
  <p>How can I reduce the width of the button programmatically?</p>