片段未加载底部导航视图

嗨，我正在制作应用程序，我使用底部导航来浏览片段，但我的问题是当我单击应用程序加载时，片段在框架布局中不可见，即使当我单击底部导航时，它也只显示下面的空白页面是我的主要活动代码我在谷歌上搜索过这个问题，但看起来我是唯一一个遇到这个问题的人

 public class MainActivity extends AppCompatActivity implements BottomNavigationView.OnNavigationItemSelectedListener{
       FrameLayout frameLayout;
       FragmentManager fragmentManager;
       Fragment fragment;
       BottomNavigationView bottomNavigationView;


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


        fragmentManager = getSupportFragmentManager();

        if(findViewById(R.id.frame_container)!=null){
            if(savedInstanceState!=null){

                return;
            }

        }
        bottomNavigationView = (BottomNavigationView) findViewById(R.id.bottom_nav);
        //bottomNavigationView.setOnNavigationItemSelectedListener(this);
        //final SeekBar sb = (SeekBar) findViewById(R.id.sb);

    }


    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        Fragment fragment = null;
        switch (item.getItemId()){
            case R.id.navigation_home:
                fragment = new HomeFragment();
                loadFragment(fragment);
                return true;
            case R.id.navigation_feed:
                fragment = new FeedFragment();
                loadFragment(fragment);
                return true;
            case R.id.navigation_event:
                fragment = new EventsFragment();
                loadFragment(fragment);
                return true;
            case R.id.navigation_nearby:
                fragment = new NearbyFragment();
                loadFragment(fragment);
                return true;
            case R.id.navigation_profile:
                fragment = new ProfileFragment();
                loadFragment(fragment);
                return true;
        }

        return false;
    }


    private void loadFragment(Fragment fragment) {
        // load fragment
        //HomeFragment homeFragment = new HomeFragment();
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        transaction.replace(R.id.frame_container, fragment,null);
       // transaction.addToBackStack(null);
      /* Comment this line and it should work!*/
        //transaction.addToBackStack(null);
        transaction.commit();
    }

}

和我的 xml

<LinearLayout android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:fillViewport="true"
    android:fitsSystemWindows="true"
    android:orientation="vertical"
    xmlns:android="http://schemas.android.com/apk/res/android">

<FrameLayout
    android:id="@+id/frame_container"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1">

</FrameLayout>
    <android.support.design.widget.BottomNavigationView
        android:id="@+id/bottom_nav"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:menu="@menu/navigation"
        android:backgroundTint="@color/nav_color"
        app:itemIconTint="@color/bottom_nav_fore_color"
        android:background="?android:attr/windowBackground"
        android:layout_weight="0.1">

    </android.support.design.widget.BottomNavigationView>
</LinearLayout>