Kotlin 在 Android Navigation Architecture 中，如何判断当前 Fragment 是否是最后一个

I need to display custom AlertDialog, but only when there are no more fragments after calling NavController.navigateUp(). My current code does something similar, but there is a bug to it:

override fun onBackPressed() {

    if (navController.navigateUp()) {
        return
    }

    showQuitDialog()
}

This somewhat works, but if I cancel the AlertDialog and don't quit the app, navController already navigated up to NavGraph root, which is not the fragment in which I was when AlertDialog appeared. So, if I try to use any navigation action from that fragment, I get error:

java.lang.IllegalArgumentException: navigation destination com.example.test/actionNavigateToNextFragment is unknown to this NavController

如果我可以访问NavController中的mBackStack字段，这个问题就可以解决，但是它是package private，并且我不能在当前项目中使用反射.但是如果它是public，我会这样使用它:

override fun onBackPressed() {

    // 2 because first one is NavGraph, second one is last fragment on the stack
    if(navController.mBackStack.size > 2) { 
        navController.navigateUp()
        return
    }

    showQuitDialog()
}

有没有办法做到这一点而不反思？