试试看：

if let vc = UIApplication.shared.delegate?.window??.rootViewController  as? FlutterViewController {
  interstitial.show(withPresenting: vc)
}

这段代码对我很有效。参考image_picker插件。

public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    switch call.method {
    case "startTestActivity":
        let vc = YourViewController()
        vc.modalPresentationStyle = .fullScreen;
        viewController(with: nil)?.present(vc, animated: true, completion: nil)
        result(true)
    default:
        result(FlutterMethodNotImplemented)
    }
}

func viewController(with window: UIWindow?) -> UIViewController? {
    var windowToUse = window
    if windowToUse == nil {
        for window in UIApplication.shared.windows {
            if window.isKeyWindow {
                windowToUse = window
                break
            }
        }
    }
    
    var topController = windowToUse?.rootViewController
    while ((topController?.presentedViewController) != nil) {
        topController = topController?.presentedViewController
    }
    return topController
}