Android Studio碎片，应用程序崩溃

Question

我正在创建一个使用BottomNavigationView切换片段的应用。每次我想添加或切换片段时应用程序崩溃。这是我的MainActivity的java代码。

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {

    FragmentManager fM = getSupportFragmentManager();
    FragmentTransaction fT = fM.beginTransaction();

    switch (item.getItemId()) {
        case R.id.nav_archive:
            fT.replace(R.id.content, new ArchiveFragment()).commit();
            fT.addToBackStack(null);
            return true;
        case R.id.nav_memo:
            fT.replace(R.id.content, new MemoFragment()).commit();
            fT.addToBackStack(null);
            return true;
        case R.id.nav_settings:
            fT.replace(R.id.content, new SettingsFragment()).commit();
            fT.addToBackStack(null);
            return true;
    }
    return false;
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_memo);
    MemoFragment memofragment = new MemoFragment();
    FragmentManager fM = getSupportFragmentManager();
    FragmentTransaction fT = fM.beginTransaction();
    setContentView(R.layout.activity_memo);
    fT.add(R.id.content, memofragment).commit();


    BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
}