How to get UINavigationController in Flutter iOS plugin (swift)

I have a third party ios native framework and would like to call it in my Flutter project.

Therefore I would like to write a flutter plugin.

However the framework need a UINavigationController as a parameter and I dont know how to
get the UINavigationController inside my Plugin.swift.

I face the similar situation in android side that I would like to get the activity/context for the android plugin. And fortunately I found the solution in here

Now I am looking for iOS side and want to know how to get UINavigationController in iOS plugin.swift similar to the android side above.

I have tried below code but I cannot get the UINavigationController.

public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
  if let viewController = UIApplication.shared.windows.first?.rootViewController as? UINavigationController {
    // do my stuff
  } else {
    result(FlutterError(code: "-1", message: "cant convert root view controller", details: nil))
  }
}

It always show “cant convert root view controller”.

Thanks