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I am trying out this qeReader lib in my app. I have an activity, where I embedd a SurfaceFragment. Here the fragment_surface layout:

<LinearLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 android:orientation="vertical"
 android:layout_width="match_parent"
 android:layout_height="match_parent">

 <SurfaceView
 android:id="@+id/surface_view"
 android:layout_width="300dp"
 android:layout_height="300dp"/>
</LinearLayout>

and my SurfaceFragment :

 override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? 
 return inflater.inflate(R.layout.fragment_surface, container, false)

and in my activity I call:

val fragmentManager = this.supportFragmentManager
val fragmentTransaction = fragmentManager.beginTransaction()
fragmentTransaction.replace(R.id.framelayout_qr, SurfaceFragment())
fragmentTransaction.commit()

and in my activities layout:

<FrameLayout
 android:id="@+id/framelayout_qr"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 app:layout_constraintTop_toTopOf="parent"
 app:layout_constraintLeft_toLeftOf="parent"
 app:layout_constraintRight_toRightOf="parent"
 app:layout_constraintBottom_toBottomOf="parent"
/>

Later on I have to access from my activity the surface_view, but this crashes:

Caused by: java.lang.IllegalStateException: surface_view must not be
null

Q1: All my R are marked red and labeled as Unresolved reference: R , although they are imported in the code. But I can run the app, until crash. Why is it marked red, but obviously imported?

Q2: I've read that I do not need those findViewById() things in Kotlin anymore? Why is surface_view null from the activity, although it is replaced in the FragmentTransaction?

Q1: R is a reference to the set of resources that are generated when you "build" the app. The first time it won't exist, then subsequent times it will exist. This gets reset every time you "clean" the app. Pressing "rebuild" will run a clean first, then a build. Although this is the way the Android resource system is designed, the Android Studio inspection isn't smart enough to know this the first time round, so it mistakenly thinks you'll get a compile time error. If this persists, try invaliding caches in Android Studio, doing a gradle sync, then rebuilding the project.

Q2: The Kotlin Android Extensions plugin is a feature that came out in the last couple of years that avoids having to using findViewById(...) to access views and widgets. You can only access views and widgets that are declared in an xml file from the Kotlin file which you use setContentView(layout_xml_file) in.