Problems with fragments and bottom navigation bar

I'm starting in the world of android and kotlin, and I'm having a problem that when I click on an item in the bottom nav bar, instead of going through fragment, I close the app. I sent them the code of the classes and fragments that I generated to see if they can give me a hand.

LoginActivity

private fun attemptLogin() {
    if (mAuthTask != null) {
        return
    }

    // Reset errors.
    email.error = null
    password.error = null

    // Store values at the time of the login attempt.
    val emailStr = email.text.toString()
    val passwordStr = password.text.toString()

    var cancel = false
    var focusView: View? = null

    // Check for a valid password, if the user entered one.
    if (!TextUtils.isEmpty(passwordStr) && !isPasswordValid(passwordStr)) {
        password.error = getString(R.string.error_invalid_password)
        focusView = password
        cancel = true
    }

    // Check for a valid email address.
    if (TextUtils.isEmpty(emailStr)) {
        email.error = getString(R.string.error_field_required)
        focusView = email
        cancel = true
    } else if (!isEmailValid(emailStr)) {
        email.error = getString(R.string.error_invalid_email)
        focusView = email
        cancel = true
    }

    if (cancel) {
        // There was an error; don't attempt login and focus the first
        // form field with an error.
        focusView?.requestFocus()
    } else {
        // Show a progress spinner, and kick off a background task to
        // perform the user login attempt.
        showProgress(true)
        //mAuthTask = UserLoginTask(emailStr, passwordStr)
        //mAuthTask!!.execute(null as Void?)

        mAuth.signInWithEmailAndPassword(emailStr, passwordStr).addOnCompleteListener { task ->
            if(task.isSuccessful){
                startActivity(Intent(this, MainActivity::class.java))
                this.finish()
            }
            else {
                Toast.makeText(this, "Usuario o contraseña incorrectos,\nintente de nuevo",
                        Toast.LENGTH_LONG).show()
                startActivity(Intent(this, LoginActivity::class.java))
                this.finish()
            }
        }
    }
}

MainActivity

class MainActivity: AppCompatActivity () {

private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
    when (item.itemId) {
        R.id.navigation_profile -> {
            message.setText(R.string.title_profile)

            val profileFragment = ProfileFragment()
            openFragment(profileFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.navigation_map -> {
            message.setText(R.string.title_map)

            val mapFragment = MapFragment()
            openFragment(mapFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.navigation_notifications -> {
            message.setText(R.string.title_notifications)

            val notificationFragment = NotificationsFragment()
            openFragment(notificationFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.navigation_qr_scan -> {
            message.setText(R.string.title_qr_scan)

            val qrScanFragment = QrScanFragment()
            openFragment(qrScanFragment)
            return@OnNavigationItemSelectedListener true
        }
    }
    false
}

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

    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)
}

private fun openFragment(fragment: Fragment) {
    val transaction = supportFragmentManager.beginTransaction()

    transaction.replace(R.id.container, fragment)
            .setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE)
            .addToBackStack(null)
            .commit()
}

}

NotificationFragment

class NotificationsFragment: Fragment () {

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                          savedInstanceState: Bundle?): View? {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_notifications, container, false)
}