     
     var viewModel: SessionViewModel? = nil {
         didSet {
+            menuButtonsContainer.isHidden = (viewModel == nil)
+            
             self.shelfController.viewModel = viewModel
             self.summaryController.viewModel = viewModel
             

    private lazy var menuButtonsContainer: WWDCBottomBorderView = {
        let v = WWDCBottomBorderView()
        
        v.wantsLayer = true
        
        v.heightAnchor.constraint(equalToConstant: 50).isActive = true

        b.state = NSOffState
        b.target = self
        b.action = #selector(tabButtonAction(_:))
        b.isHidden = true
        
        return b
    }()
