This is because findViewById() searches in the activity_main layout, while the button is located in the fragment's layout fragment_main.
Move that piece of code in the onCreateView() method of the fragment:
//...
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
Button buttonClick = (Button)rootView.findViewById(R.id.button);
buttonClick.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
onButtonClick((Button) view);
}
});
Notice that now you access it through rootView view:
Button buttonClick = (Button)rootView.findViewById(R.id.button);
otherwise you would get again NullPointerException.