Jetpack-Navigation xxx does not

布局：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/my_nav_graph" />
</androidx.constraintlayout.widget.ConstraintLayout>

代码：

  @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        NavController controller = Navigation.findNavController(this,R.id.fragment);
        NavigationUI.setupActionBarWithNavController(this,controller);
    }

原因：
使用 FragmentContainerView 创建 NavHostFragment，或通过 FragmentTransaction 手动将 NavHostFragment 添加到您的 Activity 时，尝试通过 Navigation.findNavController(Activity, @IdRes int) 检索 Activity 的 onCreate() 中的 NavController 将失败。

修改方法：
直接从 NavHostFragment 检索 NavController。

NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.fragment);
        NavController controller = navHostFragment.getNavController();