title: Facebook app invitation iOS SDK can't display the Invitation VC
body: <p>So this's my code for app invite : </p>
  
  <pre><code>private func inviteFriends() {
  
      let content = FBSDKAppInviteContent()
      content.appLinkURL = URL(string: "...")
      content.appInvitePreviewImageURL    = URL(string: "...")
      FBSDKAppInviteDialog.show(from: self, with: content, delegate: nil)
  }
  </code></pre>
  
  <p>This code works fine but if I try to add the promotional code like this  : </p>
  
  <pre><code>private func inviteFriends() {
  
      let content = FBSDKAppInviteContent()
      content.appLinkURL = URL(string: "...")
      content.appInvitePreviewImageURL    = URL(string: "...")
      content.promotionCode = "preview"
      content.promotionText = "Use the *preview* code to unlock the app"
      FBSDKAppInviteDialog.show(from: self, with: content, delegate: nil)
  }
  </code></pre>
  
  <p>The the invite VC is not shown any more (the function is called but nothing is showing). What did I missed here ?</p>