NullPointerException when trying to access views in a Kotlin fragment

Kotlin synthetic properties are not magic and work in a very simple way. When you access btn_K, it calls for getView().findViewById(R.id.btn_K).

The problem is that you are accessing it too soon. getView() returns null in onCreateView. Try doing it in the onViewCreated method:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    btn_K.setOnClickListener { Log.d(TAG, "onViewCreated(): hello world"); }
}