Tablayout + Viewpager

initViewPager

//with Icon in an activity
private fun setupViewPager() {
  val pageAdapter = ViewPagerAdapter(supportFragmentManager)
  user_profile_viewpager.adapter = pageAdapter
  user_profile_tabLayout.setupWithViewPager(user_profile_viewpager)
  user_profile_tabLayout.getTabAt(0)?.setIcon(R.drawable.ic_list_grey_24dp)
  user_profile_tabLayout.getTabAt(1)?.setIcon(R.drawable.ic_favorite_border_grey_24dp)
  user_profile_tabLayout.getTabAt(2)?.setIcon(R.drawable.ic_add_to_wishlist)
  
}

PagerAdapter.kt

class PagerAdapter(fragmentManager: FragmentManager) : FragmentStatePagerAdapter(fragmentManager) {

    // Returns total number of pages
    override fun getCount(): Int {
        return NUM_ITEMS
    }

    // Returns the fragment to display for that page
    override fun getItem(position: Int): Fragment? {
        when (position) {
            0 // Fragment # 0 - This will show FirstFragment
            -> return UserPostsFragment.newInstance()
            1 // Fragment # 0 - This will show FirstFragment different title
            -> return UserRecommendedFragment.newInstance()
            2 // Fragment # 1 - This will show SecondFragment
            -> return UserWishListFragment.newInstance()
            else -> return null
        }
    }

    // Returns the page title for the top indicator
    //Return null if only want icon
    override fun getPageTitle(position: Int): CharSequence? {
        return null
    }

    companion object {
        private val NUM_ITEMS = 3
    }

}

setupViewPager

//without Icon in a fragment
private fun setupViewPager() {
        childFragmentManager.let {
            val adapter = ViewPagerAdapter(it).apply {
                addFrag(Fragment1(), getString(R.string.all_posts))
                addFrag(Fragment2.newInstance(), getString(R.string.nearby_posts))
                addFrag(Fragment3(), getString(R.string.following))
            }
            fragment_share_viewpager.adapter = adapter
            // by default value is 1
            fragment_share_viewpager.offscreenPageLimit = 2
            // only needed if you want to be aware of fragment curreclty being viewed
            fragment_share_viewpager.addOnPageChangeListener(object : androidx.viewpager.widget.ViewPager.OnPageChangeListener {
                override fun onPageScrollStateChanged(p0: Int) {

                }

                override fun onPageScrolled(p0: Int, p1: Float, p2: Int) {

                }

                override fun onPageSelected(position: Int) {
                    
                }
            })
        }
    }

ViewPagerAdapter.kt

class ViewPagerAdapter(manager: FragmentManager) : FragmentPagerAdapter(manager) {

    val mFragmentList = ArrayList<Fragment>()
    private val mFragmentTitleList = ArrayList<String>()

    fun addFrag(fragment: Fragment, title: String) {
        mFragmentList.add(fragment)
        mFragmentTitleList.add(title)
    }

    override fun getItem(position: Int): Fragment {
        return mFragmentList[position]
    }

    override fun getCount(): Int {
        return mFragmentList.size
    }

    override fun getPageTitle(position: Int): CharSequence? {
        return mFragmentTitleList[position]
    }
}

ViewPagerAdapter implementation is wrong. It should be just like your FragmentStatePagerAdapter implementation.

Never store a list of Fragments inside a FragmentPagerAdapter. See https://stackoverflow.com/questions/58594982/fragment-has-not-been-attached-yet/58605339#58605339