Как обратиться к ViewPager из динамически назначенного Fragment?

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main_content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".FunBoxStore">

    <android.support.v4.view.ViewPager
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />

</android.support.design.widget.CoordinatorLayout>

class Shop_fragment : Fragment() {
    val TAG = "Shop"
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_shop, null)
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main_content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".FunBoxStore">

    <LinearLayout
        android:id="@+id/active_fragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_above="@id/switch_for_fragment" />

    <LinearLayout
        android:id="@+id/switch_for_fragment"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_alignParentBottom="true">

        <Button
            android:id="@+id/shop_btn"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/shop_btn" />

        <Button
            android:id="@+id/store_btn"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/store_btn" />

    </LinearLayout>
</RelativeLayout>

class FunBoxStore : AppCompatActivity() {

    private val shop_fragment: Shop_fragment = Shop_fragment()
    private val store_fragment: Store_fragment = Store_fragment()
    private var manager: FragmentManager? = null
    private var transaction: FragmentTransaction? = null
    private var mSectionsPagerAdapter: SectionsPagerAdapter? = null
    private val products = ArrayList<Product>()
    private val products_showed = ArrayList<Product>()
    private val blank_product = Product("Товаров больше нет",0f,0)

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

        manager=supportFragmentManager
        transaction = manager?.beginTransaction()
        transaction?.add(R.id.active_fragment,shop_fragment,shop_fragment.TAG)
        transaction?.commit()

        shop_btn.setOnClickListener{
            transaction = manager?.beginTransaction()
            transaction?.replace(R.id.active_fragment,shop_fragment,shop_fragment.TAG)
            transaction?.commit()
        }

        mSectionsPagerAdapter = SectionsPagerAdapter(supportFragmentManager)

        //Попытка установить adapter через findViewById

        //val viewPager = findViewById<ViewPager>(R.id.container)
        //viewPager.adapter = mSectionsPagerAdapter

        //Попытка установить adapter через findFragmentByTag (в if не входит без него виснет)

        /*if(manager?.findFragmentByTag(shop_fragment.TAG) is Shop_fragment){
            (manager?.findFragmentByTag(shop_fragment.TAG) as Shop_fragment).container.adapter = mSectionsPagerAdapter
            Log.d("myLogs", "нашелся")
        }*/

        //Попытка установить adapter через прямое обращение к экземпляру класса

        //shop_fragment.container.adapter = mSectionsPagerAdapter

        //Способ напрямую без фрагментов работает если ViewPager разместить на activity_fun_box_store.xml

        //container.adapter = mSectionsPagerAdapter

        products.add(Product("Apple iPod touch 5 32Gb", 8888f, 5))
        products.add(Product("Samsung Galaxy S Duos S7562", 7230f, 5))
        products.add(Product("Canon EOS 600D Kit", 15659f, 0))

        for (product in products) {
            if (product.merch_amt>0) {
                products_showed.add(product)
            }
        }

        //Попытка обратиться к adapter через findViewById

        //viewPager.adapter?.notifyDataSetChanged()

        //Попытка обратиться к adapter через findFragmentByTag (в if не входит без него виснет)

        /*if(manager?.findFragmentByTag(shop_fragment.TAG) is Shop_fragment){
            (manager?.findFragmentByTag(shop_fragment.TAG) as Shop_fragment).container.adapter?.notifyDataSetChanged()
            Log.d("myLogs", "нашелся")
        }*/

        //Попытка обратиться к adapter через прямое обращение к экземпляру класса

        //shop_fragment.container.adapter?.notifyDataSetChanged()

        //Способ напрямую без фрагментов работает если ViewPager разместить на activity_fun_box_store.xml

        //container.adapter = mSectionsPagerAdapter
    }

    override fun onCreateOptionsMenu(menu: Menu): Boolean {
        // Inflate the menu; this adds items to the action bar if it is present.
        menuInflater.inflate(R.menu.menu_fun_box_store, menu)
        return true
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        val id = item.itemId
        if (id == R.id.action_settings) {
            return true
        }
        return super.onOptionsItemSelected(item)
    }

    inner class SectionsPagerAdapter(fm: FragmentManager) : FragmentPagerAdapter(fm) {

        override fun getItem(position: Int): Fragment {
            return if (products_showed.size > 0) {
                PlaceholderFragment.newInstance(position + 1, products_showed[position])
            }else{
                PlaceholderFragment.newInstance(position+1,blank_product)
            }
        }

        override fun getCount(): Int {
            Log.d("myLogs", "fuck"+products_showed.size.toString())
            return if (products_showed.size > 0) {
                products_showed.size
            }else{
                1
            }
        }
    }

    class PlaceholderFragment : Fragment() {

        override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                                  savedInstanceState: Bundle?): View? {
            val rootView = inflater.inflate(R.layout.fragment_fun_box_store, container, false)
            rootView.name.text =  arguments?.getString("MERCH_NAME")
            rootView.price.text =  arguments?.getFloat("MERCH_PRICE").toString()
            rootView.amt.text =  arguments?.getInt("MERCH_AMT").toString()
                    //getString(R.string.section_format, arguments?.getInt(ARG_SECTION_NUMBER))
            return rootView
        }

        companion object {

            private val ARG_SECTION_NUMBER = "section_number"

            fun newInstance(sectionNumber: Int, product:Product): PlaceholderFragment {
                val fragment = PlaceholderFragment()
                val args = Bundle()
                args.putInt(ARG_SECTION_NUMBER, sectionNumber)
                args.putString("MERCH_NAME",product.merch_name)
                args.putFloat("MERCH_PRICE",product.merch_price)
                args.putInt("MERCH_AMT",product.merch_amt)
                fragment.arguments = args
                return fragment
            }
        }
    }
}