问为什么getSupportFragmentManager()返回NULL

EN

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

private TabLayout tabLayout;
private ViewPager viewPager;
private TabItem tab1, tab2;
public PageAdapter pageAdapter;

private DrawerLayout drawer;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Toolbar toolbar = findViewById(R.id.toolbar);

    drawer = findViewById(R.id.drawer_layout);
    NavigationView navigationView = findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    if(savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new HomeFragment()).commit();
        navigationView.setCheckedItem(R.id.nav_home);
    }

    tabLayout = (TabLayout) findViewById(R.id.tabLayout);
    tab1 = (TabItem) findViewById(R.id.tab1);
    tab2 = (TabItem) findViewById(R.id.tab2);
    viewPager = (ViewPager) findViewById(R.id.viewPager);


    pageAdapter = new PageAdapter(getSupportFragmentManager(), tabLayout.getTabCount());
    viewPager.setAdapter(pageAdapter);

    tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            viewPager.setCurrentItem(tab.getPosition());

            if(tab.getPosition() == 0)
            {
                pageAdapter.notifyDataSetChanged();
            }
            else if(tab.getPosition() == 1)
            {
                pageAdapter.notifyDataSetChanged();
            }
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }

<androidx.drawerlayout.widget.DrawerLayout
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"
android:id="@+id/drawer_layout"
android:fitsSystemWindows="false"
tools:context=".MainActivity"
tools:openDrawer="start">

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@color/green"
        android:minHeight="?attr/actionBarSize"
        android:theme="?attr/actionBarTheme" />

    <FrameLayout
        android:id="@+id/fragment_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>


</LinearLayout>

<com.google.android.material.navigation.NavigationView
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:background="#5E5E5E"
    android:id="@+id/nav_view"
    app:headerLayout="@layout/main_nav_drawer"
    app:menu="@menu/drawer_menu"
    app:itemTextColor="@android:color/white"
    app:itemTextAppearance="@style/NavText"/>

<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:theme="@style/Theme.AppCompat.Light.NoActionBar"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
app:layout_constraintBottom_toTopOf="parent"
android:orientation="vertical"
tools:context=".MainActivity"
android:background="@color/beige">

<com.google.android.material.tabs.TabLayout
    android:id="@+id/tabLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:tabBackground="@drawable/tab_background"
    app:tabIndicatorColor="@color/black"
    app:tabSelectedTextColor="@color/black">

    <com.google.android.material.tabs.TabItem
        android:id="@+id/tab1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/recipes_tab" />

    <com.google.android.material.tabs.TabItem
        android:id="@+id/tab2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/fav_tab" />
</com.google.android.material.tabs.TabLayout>

<androidx.viewpager.widget.ViewPager
    android:id="@+id/viewPager"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

</androidx.viewpager.widget.ViewPager>