Button.setOnClickListener во фрагменте не работает

это мой макет -> fragment_three.xml

<ScrollView
xmlns:app = "http://schemas.android.com/apk/res-auto"
xmlns:android = "http://schemas.android.com/apk/res/android"
xmlns:tools = "http://schemas.android.com/tools"
android:layout_width = "match_parent"
android:layout_height = "match_parent"
android:background = "@drawable/supportback">
    <LinearLayout
        android:layout_width = "match_parent"
        android:layout_height = "wrap_content"
        android:orientation = "vertical">
        //some Textviews here...
          <Button
            android:id = "@+id/sendButton"
            android:layout_width = "58dp"
            android:layout_height = "51dp"
            android:layout_gravity = "right"
            android:layout_marginLeft = "20dp"
            android:layout_marginRight = "20dp"
            android:clickable = "true"
            android:background = "@drawable/send_email"/>
    </LinearLayout>
</ScrollView>

а это мой фрагмент:

class fragmentC : Fragment()  {
companion object {
    fun newInstance():Fragment {
        var fb :fragmentC = fragmentC()
        return fb 
}}

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    val rootView = inflater.inflate(R.layout.fragment_three,container,false)
    val btn = rootView.findViewById(R.id.sendButton) as Button
    btn.setOnClickListener {
        Toast.makeText(context,"test",Toast.LENGTH_LONG).show()
    }
    return rootView
}}

нет ошибок, нет исключений, ничего плохого в logcat. Я пока не смог найти проблему, пожалуйста, помогите?

edit1

это макет, в котором должен работать фрагмент:

<?xml version = "1.0" encoding = "utf-8"?>
<RelativeLayout xmlns:android = "http://schemas.android.com/apk/res/android"
android:layout_width = "match_parent"
android:layout_height = "match_parent"
xmlns:tools = "http://schemas.android.com/tools"
xmlns:app = "http://schemas.android.com/apk/res-auto"
tools:context = ".MainActivity"
android:id = "@+id/activity_main">

<FrameLayout
    android:id = "@+id/frame_layout"
    android:layout_width = "match_parent"
    android:layout_height = "match_parent"
    android:layout_above = "@+id/navigation"
    android:animateLayoutChanges = "true">
</FrameLayout>

<android.support.design.widget.BottomNavigationView
    android:id = "@+id/navigation"
    android:layout_width = "match_parent"
    android:layout_height = "wrap_content"
    android:layout_alignParentBottom = "true"
    android:background = "#389486"
    app:menu = "@menu/bottom_view">
</android.support.design.widget.BottomNavigationView>
</RelativeLayout>

P.S: есть два других фрагмента, но сейчас он пустой, я его еще не трогал

edit2 основной вид деятельности

class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
        navigation.setOnNavigationItemSelectedListener (object : BottomNavigationView.OnNavigationItemSelectedListener{
            override fun onNavigationItemSelected(item: MenuItem): Boolean {
                when(item.itemId){
                    R.id.bot_list ->{
                    drawerLayout.openDrawer(Gravity.START)
                    }
                    R.id.bot_home ->{
                            var ft : FragmentTransaction = supportFragmentManager.beginTransaction()
                            ft.replace(R.id.frame_layout,fragmentA.newInstance())
                            ft.commit()
                    }
                    R.id.bot_cart ->{
                        var ft : FragmentTransaction = supportFragmentManager.beginTransaction()
                            ft.replace(R.id.frame_layout,fragmentB.newInstance())
                            ft.commit() }
                    R.id.bot_message ->{
                        //here is the required fragment
                        var ft : FragmentTransaction = supportFragmentManager.beginTransaction()
                        ft.replace(R.id.frame_layout,fragmentC.newInstance())
                        ft.commit() }
                }
                return true
            }
        })
    var ft : FragmentTransaction = supportFragmentManager.beginTransaction()
    ft.replace(R.id.frame_layout,fragmentA.newInstance())
    ft.commit()
}

}