Android 如何打开BottomNavigationView'；活动中的碎片是什么？

我想调用方法 showNewsPageTime（） 从AlertBoxActivity.kt中的MainFragment.kt打开 新闻片段， 但应用程序已经停止

    package kz.app

    import android.support.design.widget.BottomNavigationView
    import android.annotation.SuppressLint
    import android.os.Bundle
    import android.support.v7.app.AppCompatActivity
    import android.support.v4.app.FragmentManager;
    import kotlinx.android.synthetic.main.fragment_main.*
    import kotlinx.android.synthetic.main.fragment_parking_payment.*
    import kotlinx.android.synthetic.main.fragment_shopping.*
    import kz.app.api.entity.PromoAction
    import kz.app.model.PromoActionViewModel
    import kz.app.fragment.NewsFragment
    import kz.app.fragment.PromoDetailsFragment
    import kz.app.fragment.MainFragment
    import kz.app.fragment.ShoppingDetailsFragment
    import kz.app.model.PromoDetailsViewModel
    import android.widget.FrameLayout;
    import android.R

    class AlertBoxActivity : AppCompatActivity() {

        override fun onCreate(savedInstanceState: Bundle?) {
            val fragmentManager = supportFragmentManager
            super.onCreate(savedInstanceState)
            val obj = MainFragment()
            obj.showNewsPageTime()
            if (savedInstanceState == null) {

                getSupportFragmentManager().beginTransaction()
                        .add(android.R.id.content, obj)
                        .commit()

MainFragment.kt

（

BottomNavigationView

位于此处。我想从此处打开

NewsFragment

页面，因为如果直接打开，底部导航将消失。）

    package kz.app

    import android.support.design.widget.BottomNavigationView
    import android.annotation.SuppressLint
    import android.os.Bundle
    import android.support.v7.app.AppCompatActivity
    import android.support.v4.app.FragmentManager;
    import kotlinx.android.synthetic.main.fragment_main.*
    import kotlinx.android.synthetic.main.fragment_parking_payment.*
    import kotlinx.android.synthetic.main.fragment_shopping.*
    import kz.app.api.entity.PromoAction
    import kz.app.model.PromoActionViewModel
    import kz.app.fragment.NewsFragment
    import kz.app.fragment.PromoDetailsFragment
    import kz.app.fragment.MainFragment
    import kz.app.fragment.ShoppingDetailsFragment
    import kz.app.model.PromoDetailsViewModel
    import android.widget.FrameLayout;
    import android.R

    class AlertBoxActivity : AppCompatActivity() {

        override fun onCreate(savedInstanceState: Bundle?) {
            val fragmentManager = supportFragmentManager
            super.onCreate(savedInstanceState)
            val obj = MainFragment()
            obj.showNewsPageTime()
            if (savedInstanceState == null) {

                getSupportFragmentManager().beginTransaction()
                        .add(android.R.id.content, obj)
                        .commit()