Android SDK error: Trying instantiate a class that is not a fragment

Question

I am hardly trying to create a simple application with a top menu and a changeable view below (by pressing the buttons in the menu fragment we change the view of the fragment be

Answer 1

In my case it turned out, I was doing stuff in onCreate in the wrong order:

setContentView(R.layout.activity_qr_code_scan);
super.onCreate(savedInstanceState);

instead of

super.onCreate(savedInstanceState);
setContentView(R.layout.activity_qr_code_scan);