After migrate to AndroidX: "Unresolved reference: ..."

I got some errors in my application and found this solution: Click

Now, nothing works. When I run the project, I get errors like

Unresolved reference: adapter   
Unresolved reference: currentItem   
Unresolved reference: addOnPageChangeListener   

Those errors coming from the MainActivity. I added there a ViewPager a few days ago. Since I migrated to androidX, the viewpager shows a lot of errors.

lateinit var myAdapter: myPagerAdapter

// view_pager from activity_main.xml

view_pager.adapter = myAdapter
    btn_next.setOnClickListener {
        view_pager.currentItem++
    }

view_pager.addOnPageChangeListener(object : ViewPager.OnPageChangeListener{
        override fun onPageScrollStateChanged(p0: Int) {

        }

I found the solution. Check the xml-file and change

<androidx.support.v4.view.ViewPager
...

to:

<androidx.viewpager.widget.ViewPager
...