     
     override func willTransitionToTraitCollection(newCollection: UITraitCollection, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
         super.willTransitionToTraitCollection(newCollection, withTransitionCoordinator: coordinator)
+        guard let bvc = self.bvc else {
+            return
+        }
         tableView.snp_updateConstraints { make in
             make.bottom.equalTo(self.view).offset(bvc.shouldShowFooterForTraitCollection(newCollection) ? -UIConstants.ToolbarHeight : 0)
         }

    }()
    
    var tabManager: TabManager!
    weak var bvc: BrowserViewController!
    var listData = [(item:WKBackForwardListItem, type:BackForwardType)]()
    
    var tableHeight: CGFloat

    
    override func viewDidLoad() {
        super.viewDidLoad()
        guard let bvc = self.bvc else {
            return
        }
        view.addSubview(shadow)
        view.addSubview(tableView)
        tableView.snp_makeConstraints { make in
