IllegalStateException: RecyclerView is null inside of Fragment within NavigationDrawer activity

You have not initialized the RecyclerView in MainFragment.kt file. You have to initialize it before the below line:

mainMenu.layoutManager = LinearLayoutManager(this.context)

You can initialize the RecyclerView by the below line:

var mainMenu = findViewById(R.id.mainMenu) as RecyclerView

You have to change it as per your need.

For anyone that follow the brilliant clue of @Avijit Karmakar, getting in count another discussion, (RecyclerView error: No adapter attached; skipping layout) I had to include :

class WikiFragment : Fragment() {
    val paginas: ArrayList<String> = ArrayList()    

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
var listItems : View  = inflater.inflate(R.layout.fragment_wiki, container, false)

var rv_frag_wiki = listItems.findViewById<View>(R.id.rv_frag_wiki) as RecyclerView


 rv_frag_wiki.layoutManager = LinearLayoutManager(this.context)
        rv_frag_wiki.adapter = PaginasAdapter(paginas, this.context)
        
  return listItems 
  

Since you are using kotlin you can have advantage of synthetics where you don't need to do var rv_frag_wiki = listItems.findViewById<View>(R.id.rv_frag_wiki) as RecyclerView

Rather import synthetics via plugin Step 1 :

apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

Step 2 :

Import sythetic in MainFragment

import kotlinx.android.synthetic.main.content_menu.* //

Step 3 :

Finally in MainFragment

Shift this code of recyclerView here

 override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        rvList.layoutManager = LinearLayoutManager(context, 
        LinearLayoutManager.VERTICAL, false)
        rvList.adapter = academyAdapter

    }