Android. Fragment getActivity() sometimes returns null

In developer console error reports sometimes I see reports with NPE issue. I do not understand what is wrong with my code. On emulator and my device application works good without forcecloses, however some users get NullPointerException in fragment class when the getActivity() method is called.

Activity

pulic class MyActivity extends FragmentActivity{

    private ViewPager pager; 
    private TitlePageIndicator indicator;
    private TabsAdapter adapter;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        pager = (ViewPager) findViewById(R.id.pager);
        indicator = (TitlePageIndicator) findViewById(R.id.indicator);
        adapter = new TabsAdapter(getSupportFragmentManager(), false);

        adapter.addFragment(new FirstFragment());
        adapter.addFragment(new SecondFragment());
        indicator.notifyDataSetChanged();
        adapter.notifyDataSetChanged();

        // push first task
        FirstTask firstTask = new FirstTask(MyActivity.this);
        // set first fragment as listener
        firstTask.setTaskListener((TaskListener) adapter.getItem(0));
        firstTask.execute();
    }

    indicator.setOnPageChangeListener(new ViewPager.OnPageChangeListener()  {
        @Override
        public void onPageSelected(int position) {
            Fragment currentFragment = adapter.getItem(position);
            ((Taskable) currentFragment).executeTask();
        }

        @Override
        public void onPageScrolled(int i, float v, int i1) {}

        @Override
        public void onPageScrollStateChanged(int i) {}
    });
}

AsyncTask class

public class FirstTask extends AsyncTask{

    private TaskListener taskListener;

    ...

    @Override
    protected void onPostExecute(T result) {
        ... 
        taskListener.onTaskComplete(result);
    }   
}

Fragment class

public class FirstFragment extends Fragment immplements Taskable, TaskListener{

    public FirstFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.first_view, container, false);
    }

    @Override
    public void executeTask() {
        FirstTask firstTask = new FirstTask(MyActivity.this);
        firstTask.setTaskListener(this);
        firstTask.execute();
    }

    @Override
    public void onTaskComplete(T result) {
        // NPE is here 
        Resources res = getActivity().getResources();
        ...
    }
}

Maybe this error happens when applications resumed from background. In this case how I should handle this situation properly?