MainActivity.kt

class MainActivity : SingleFragmentActivity() {

   ...

    private val navController by lazy {
        val navHostFragment =
            supportFragmentManager.findFragmentById(R.id.fragment_container) as NavHostFragment
        navHostFragment.navController
    }
    val navigator by lazy { Navigator(navController, resources) }

   private val appBarConfigurations by lazy {
       AppBarConfiguration
           .Builder(topLevelDestinations)
           .setDrawerLayout(drawerLayout)
           .build()
   }

   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)

       setupNavigation()
   }

   /**
    * Navigation Component setup
    */
   override fun onSupportNavigateUp(): Boolean {
       return NavigationUI.navigateUp(navController, appBarConfigurations)
   }

   override fun onBackPressed() {
       if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
           drawerLayout.closeDrawer(GravityCompat.START)
       } else {
           super.onBackPressed()
       }
   }

   private fun setupNavigation() {
       // Update action bar to reflect navigation
       NavigationUI.setupActionBarWithNavController(this, navController, appBarConfigurations)

       // Handle nav drawer item clicks
       navigationView.setNavigationItemSelectedListener { menuItem ->
           menuItem.isChecked = true
           drawerLayout.closeDrawers()
           true
       }

       // Tie nav graph to items in nav drawer
       NavigationUI.setupWithNavController(navigationView, navController)
   }

   companion object {
       // The Up button will not be displayed at top-level destinations in the information hierarchy
       val topLevelDestinations = hashSetOf(
           R.id.errorFragment,
           R.id.invasiveSpeciesMapFragment,
       )
   }
}