title: OSX extension doesn't call tableView viewFor method
body: <p>I've set up a <code>Table View</code> to be displayed in an OSX extension. <br>
  I  basically copied the code from the OSX <code>TableViewController</code>, I've then created the related interface in the .xib <code>TodayViewController</code>, connected the <code>tableView</code> from the <code>File's Owner</code> to the tableview component. <br>
  I've also set <code>TodayViewController.xib</code> as <code>Main Interface</code> for the extension in the <code>General</code> panel</p>
  
  <p>The problem is that when I run the extension the method <code>numberOfRows(in tableView: NSTableView)</code> is fired correctly, but not <code>tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int)</code>. Hence my view is empty. <br></p>
  
  <p>This is the set up I have</p>
  
  <pre><code>class TodayViewController: NSViewController, NSTableViewDataSource, NSTableViewDelegate, NCWidgetProviding  {
  
      @IBOutlet weak var tableView: NSTableView!
  
      override var nibName: NSNib.Name? {
          return NSNib.Name("TodayViewController")
      }
  
      func widgetPerformUpdate(completionHandler: (@escaping (NCUpdateResult) -&gt; Void)) {
          // Update your data and prepare for a snapshot. Call completion handler when you are done
          // with NoData if nothing has changed or NewData if there is new data since the last
          // time we called you
          completionHandler(.noData)
      }
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          tableView.delegate = self
          tableView.dataSource = self
  
          tableView.headerView = nil
  
          tableView.target = self
          tableView.action = #selector(self.onItemClicked)
  
          tableView.isEnabled = true
          fetchData() // -&gt; this method contains tableView.reloadData()
      }
  
      func tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int) -&gt; NSView? { 
      // ...
      }
  
      func numberOfRows(in tableView: NSTableView) -&gt; Int {
      // ...
      }
  }
  </code></pre>
  
  <p>I think I'm missing something.</p>