If you call method getString() or getResources() or getActivity(). You get this error if your fragment hasnt been attach to the activity yet.

E.g:

private horizontalTabTitles = new String[]{getString(R.string.sort_by_time),
getString(R.string.sort_by_table), getString(R.string.sort_by_order)};

when you declare and initialize this, the fragment may not has been attached to the activity yet. hence cause the error.