我有一个AgreementActivity, 想通过findNavController回退到上一级，api是：

/**
 * Find a [NavController] given the id of a View and its containing
 * [Activity].
 *
 * Calling this on a View that is not a [NavHost] or within a [NavHost]
 * will result in an [IllegalStateException]
 */
fun Activity.findNavController(@IdRes viewId: Int): NavController

我翻了文档也不知道往里面填什么参数：
backLoginPage.setOnClickListener {
    findNavController(填什么好呢).navigateUp()
}

y有资料说填<androidx.fragment.app.FragmentContainerView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/fragment_container_view"
    app:navGraph="@navigation/nav_config"
    android:name="androidx.navigation.fragment.NavHostFragment"
    app:defaultNavHost="true"
    />

l里面的这个id,可是我试过也不对。