Nested class AsyncTask can't modify Outer Class static objects

I have a problem with Nested java class, it sees outer class object but somehow fails to modify it. I read a lot of similar questions but couldn't find solution for this one. It might be something really simple but I'm not good enough to figure it out. I have Sorter class to do some calculations in the background, I decided to use AsyncTask to perform those calculations outside of UI Thread. My class looks like this

public class Sorter
{
    private static List<Long> workingList;
    private static int _numberOfContainers, _containerSize, _timesToRepeat;
    private static Long _numbersFrom, _numbersTo, _sortingAlgorithmId;

    public Sorter(int numberOfContainers, int containerSize, Long numbersFrom, Long numbersTo,
                  int timesToRepeat, Long sortingAlgorithmId)
    {
        _numberOfContainers = numberOfContainers;
        _containerSize = containerSize;
        _numbersFrom = numbersFrom;
        _numbersTo = numbersTo;
        _timesToRepeat = timesToRepeat;
        _sortingAlgorithmId = sortingAlgorithmId;
        // perform calculations in the background
        new BackgroundCalculations().execute();
    }

    static class BackgroundCalculations extends AsyncTask<Void,Void,Void>
    {

        @Override
        protected Void doInBackground(Void... voids)
        {
            workingList = new ArrayList<>(_containerSize);
            // workingList is still null after this
            _numbersTo += 1; // to fix exclusive number range to inclusive
            Random rand = new Random();
            for (int i = 0; i < _containerSize; i++)
            {
                workingList.add((long) (rand.nextDouble() * (_numbersTo - _numbersFrom)) + _numbersFrom))
            }
            // some calc
            return null;
        }
    }


}

I tried to instantiate workingList in Sorter constructor but nested class fails to add items to workingList anyway. Any solutions? Maybe better way to implement background calculations without such problems?