底部导航栏在切换片段后被删除

Question

从对导航栏没有可见性的片段返回后，底部导航栏将被删除。做什么

这是去掉底部导航的片段

 activity?.findViewById<BottomNavigationView>(R.id.bnb)?.let {
            (it.parent as ViewGroup).removeView(it)
        }

这是主要活动

bottomNav.setOnItemSelectedListener setOnNavigationItemSelectedListener@{ item ->
            when(item.itemId){
                R.id.btn1 -> {
                    supportFragmentManager.beginTransaction().replace(R.id.nav_host_fragment_content_main, btn1Fragment()).commit()
                    return@setOnNavigationItemSelectedListener true
                }
                R.id.btn1 -> {
                    supportFragmentManager.beginTransaction().replace(R.id.nav_host_fragment_content_main, FirstFragment()).commit()
                    return@setOnNavigationItemSelectedListener true
                }
                R.id.btn2-> {
                    supportFragmentManager.beginTransaction().replace(R.id.nav_host_fragment_content_main, btn2Fragment()).commit()
                    return@setOnNavigationItemSelectedListener true
                }

                else -> false
            }
            true
        }