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

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

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