title: Add a condition to a modal push segue
body: <p>I use a navigation controller and modal show segues.
  I want to block the back button of my second view controller if [condition] isn't true, for exemple by adding an alert "You can't go back until [condition]" when the user press the back button.</p>
  
  <p>Don't know if it's possible, if someone know how to solve it !</p>
  
  <p>Thanksss</p>