Вылетает приложение Androi

Создаю чат. Отображаю чаты в фрагменте:

private fun retrieveChatList()
    {
        mUsers = ArrayList()

        val ref = FirebaseDatabase.getInstance().reference.child("Users")
        ref!!.addValueEventListener(object : ValueEventListener {
            override fun onDataChange(p0: DataSnapshot)
            {
                (mUsers as ArrayList).clear()

                for (dataSnapshot in p0.children)
                {
                    val user = dataSnapshot.getValue(User::class.java)

                    for (eachChatList in usersChatList!!)
                    {
                        if (user!!.getUID().equals(eachChatList.getId()))
                        {
                            (mUsers as ArrayList).add(user!!)
                        }
                    }
                }
                userAdapterSecond = UserAdapterSecond(context!!, (mUsers as ArrayList<User>), true)
                recycler_view_messages.adapter = userAdapterSecond
            }

            override fun onCancelled(p0: DatabaseError) {

            }
        })
    }

Код работает успешно, чаты отображаются. Но при некоторых действиях (например, регистрация или авторизация) приложение вылетает. В Logcat посмотрел, где ошибка. Не работает эта часть кода:

userAdapterSecond = UserAdapterSecond(context!!, (mUsers as ArrayList<User>), true)

Полный код фрагмента:

class MessagesFragment : Fragment() {

    private var userAdapterSecond: UserAdapterSecond? = null
    private var mUsers: List<User>? = null
    private var usersChatList: List<ChatList>? = null
    lateinit var recycler_view_messages: RecyclerView
    private var firebaseUser: FirebaseUser? = null

    private var param1: String? = null
    private var param2: String? = null

    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)
        arguments?.let {
            param1 = it.getString(ARG_PARAM1)
            param2 = it.getString(ARG_PARAM2)
        }
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        val view = inflater.inflate(R.layout.fragment_messages, container, false)


        recycler_view_messages = view.findViewById(R.id.recycler_view_messages)
        recycler_view_messages.setHasFixedSize(true)
        recycler_view_messages.layoutManager = LinearLayoutManager(context)


        firebaseUser = FirebaseAuth.getInstance().currentUser


        usersChatList = ArrayList()

        val ref = FirebaseDatabase.getInstance().reference.child("ChatList").child(firebaseUser!!.uid)
        ref!!.addValueEventListener(object : ValueEventListener{
            override fun onDataChange(p0: DataSnapshot)
            {
                (usersChatList as ArrayList).clear()

                for (dataSnapshot in p0.children)
                {
                    val chatList = dataSnapshot.getValue(ChatList::class.java)

                    (usersChatList as ArrayList).add(chatList!!)
                }
                retrieveChatList()
            }

            override fun onCancelled(p0: DatabaseError) {

            }
        })


        return view
    }

    companion object {
        fun newInstance(param1: String, param2: String) =
            MessagesFragment().apply {
                arguments = Bundle().apply {
                    putString(ARG_PARAM1, param1)
                    putString(ARG_PARAM2, param2)
                }
            }
    }

    private fun retrieveChatList()
    {
        mUsers = ArrayList()

        val ref = FirebaseDatabase.getInstance().reference.child("Users")
        ref!!.addValueEventListener(object : ValueEventListener {
            override fun onDataChange(p0: DataSnapshot)
            {
                (mUsers as ArrayList).clear()

                for (dataSnapshot in p0.children)
                {
                    val user = dataSnapshot.getValue(User::class.java)

                    for (eachChatList in usersChatList!!)
                    {
                        if (user!!.getUID().equals(eachChatList.getId()))
                        {
                            (mUsers as ArrayList).add(user!!)
                        }
                    }
                }
                userAdapterSecond = UserAdapterSecond(context!!, (mUsers as ArrayList<User>), true)
                recycler_view_messages.adapter = userAdapterSecond
            }

            override fun onCancelled(p0: DatabaseError) {

            }
        })
    

} }

В чём может быть проблема?

Лог ошибки:

2020-11-16 22:29:56.969 17231-17231/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.megapixel.trashbench, PID: 17231
    kotlin.KotlinNullPointerException
        at com.megapixel.trashbench.Fragments.MessagesFragment$retrieveChatList$1.onDataChange(MessagesFragment.kt:124)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:75)
        at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(EventRaiser.java:55)
        at android.os.Handler.handleCallback(Handler.java:888)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:213)
        at android.app.ActivityThread.main(ActivityThread.java:8178)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1101)