MainActivity.kt

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    initViews()
    initTabLayout()
    initRecycler()
    initMediator() // NEW
}

private fun initMediator() {
    TabbedListMediator(
        recyclerView,
        tabLayout,
        categories.indices.toList()
    ).attach()
}