在activity中找到fragment组件的问题

public class MainActivity extends AppCompatActivity {
    private ViewPager fragmentContainer;
    private TabLayout tabs ;
    private Fragment_main fragment_main;
    private Fragment_shop fragment_shop;
    private Fragment_me fragment_me;
    private Fragment_CitySelect fragment_citySelect;
    Button btn_CitySelect;

    @Override
    protected void onStart() {
        super.onStart();
        //初始化fragment_main中的组件
        initFragmentView();

    }

    private void initFragmentView() {
        //城市选择按钮

        View view = this.fragment_main.getView();
        btn_CitySelect = view.findViewById(R.id.btn_selecCity);
        btn_CitySelect.setOnClickListener(new Fragment_main_listener());
    }

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

    }

    private void setOnListener() {

        fragmentContainer.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabs));
        tabs.addOnTabSelectedListener(new TabLayout.ViewPagerOnTabSelectedListener(fragmentContainer));
        //main_fragment中的组件


    }

    private void initView() {

        fragmentContainer = findViewById(R.id.container);
        tabs = findViewById(R.id.tabs);
        fragment_me = Fragment_me.newInstance();
        fragment_main = Fragment_main.newInstance();
        fragment_shop = Fragment_shop.newInstance();


        final Fragment[] fragmentsList = new Fragment[]{fragment_main,fragment_shop,fragment_me};
        fragmentContainer.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
            @Override
            public Fragment getItem(int i) {
                return fragmentsList[i];
            }

            @Override
            public int getCount() {
                return fragmentsList.length;
            }
        });

    }


    private class Fragment_main_listener implements View.OnClickListener{

        @Override
        public void onClick(View v) {
            switch (v.getId()){
                case R.id.btn_selecCity:
                    fragment_main.getFragmentManager().beginTransaction().hide(fragment_main);
                    fragment_main.getChildFragmentManager().beginTransaction()
                            .add(R.id.main_fragment_main_menu, fragment_citySelect).commit();

            }
        }
    }


}

initFragmentView()函数中，fragment_main.findViewById()一直返回空值. 谢谢老师！