App crashed when I changed arrowTintColor

[168-7cm]

I wanna change arrow tintClor like below. but crashed at self.menuArrow.tintColor = color

error message is Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value

here is my code

private var menuView: BTNavigationDropdownMenu!

 private func setupDropDownMenu() {
        let items = Artist.allCases.map { $0.rawValue }
        menuView = BTNavigationDropdownMenu(navigationController: self.navigationController, containerView: self.navigationController!.view, title: items.first!, items: items)
        menuView.cellHeight = 45
        menuView.arrowPadding = 15
        menuView.animationDuration = 0.4
        menuView.cellBackgroundColor = .white
        menuView.cellTextLabelColor = .black
        menuView.cellTextLabelFont = UIFont.boldSystemFont(ofSize: 14)
        menuView.shouldKeepSelectedCellColor = true
        menuView.cellTextLabelAlignment = .left
        menuView.cellSelectionColor = .systemGray2
        menuView.checkMarkImage = nil
        //menuView.arrowTintColor = .white
        menuView.didSelectItemAtIndexHandler = { [weak self] index in
            self?.selectedArtist = Artist.allCases[index]
            self?.pagingViewController.reloadData()
        }
        self.navigationItem.titleView = menuView
    }

please help me!