title: NSTabViewController add NSToolbarItems
body: <p>I'd like to use the NSTabViewController for switching through 6 different Tabs with the toolbar style. </p>
  
  <p>All tabs have in common that they show different aspects of a Customer entity. </p>
  
  <p>Now I want to add aditional NSToolbarItems to the toolbar of the NSTabViewController? But I haven't found a way to access the toolbar. 
  I also would like to add Space between the ToolbarItems. </p>
  
  <p>Is there a way to do so?</p>
  
  <p>Or how can I add my ViewController from the Storyboard to a NSTabView without using NSTabViewController?</p>
  
  <p>Regards</p>
  
  <p>Oliver</p>
  
  <hr>
  
  <p>In the meantime I've tried another approach that I thought was more promising but lead to another strange behaviour:</p>
  
  <p>I've created a new NSViewController and put a NSTabView inside. In order to load my already existing ViewControllers I used this </p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
      let customerController = self.storyboard?.instantiateController(withIdentifier: NSStoryboard.SceneIdentifier("CustomerVCID")) as! CustomerViewController
  
      let servicesController = self.storyboard?.instantiateController(withIdentifier: NSStoryboard.SceneIdentifier("ServicesVCID")) as! ServicesController
  
      customerController.customer = self.customer
      servicesController.customer = self.customer
  
      self.tabView.tabViewItems[0].view = customerController.view
      self.tabView.tabViewItems[1].view = servicesController.view
  
  }
  </code></pre>
  
  <p>That indeed worked, but now all my NSButtons that have actions will cause my application to crash. </p>