GraphView Android

Question

I am not sure if I am missing something conceptually or if there is an error in one of my methods, but I have debugged this activity for quite some time and I must be overlooking something huge. At the end of my onCreate, I instantiate an ASyncTask to populate an ArrayList of data from a GET, and then in the onPostExecute I reset the GraphView data with the new data. My onCreate is below:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_cholestoral);
    graph = (GraphView) findViewById(R.id.graph);
    //dummy data for now, TODO: adjust dates to fit in graphical format
    HDL_series = new LineGraphSeries<>(new DataPoint[] {
            new DataPoint(0, 2),
            new DataPoint(1, 1),
            new DataPoint(2, 5),
            new DataPoint(3, 3),
            new DataPoint(4, 6),
    });
    HDL_series.setColor(Color.RED);
    HDL_series.setTitle("HDL");
    LDL_series = new LineGraphSeries<>(new DataPoint[] {
            new DataPoint(0, 7),
            new DataPoint(1, 8),
            new DataPoint(2, 5),
            new DataPoint(3, 8),
            new DataPoint(4, 7),
    });
    LDL_series.setColor(Color.CYAN);
    LDL_series.setTitle("LDL");
    graph.addSeries(HDL_series);
    graph.addSeries(LDL_series);
    //empty arraylist on page load... to be filled in ASyncTask below
    new ListCholestoralAPI().execute("http://m-health.cse.nd.edu:8000/phrService-0.0.1-SNAPSHOT/chol/chol/");

The doInBackground correctly populates my list of information, and it gets sent to the onPostExecute which must be the section that keeps crashing the application. My onPostExecute simply calls a function in my activity called initializeGraph().

public void initializeGraph() {
    //TODO: initialize graph by setting labels, axes, etc...
    HDL_series.resetData(generateDataPoints());
}

public DataPoint[] generateDataPoints() {
    dataPointArray = null;
    dataPointArray = new DataPoint[30];
    for (int i = 0; i < cholesterolInformationList.size(); i++) {
        double hdl = cholesterolInformationList.get(i).getHdl();
        dataPointArray[i] = new DataPoint(i, hdl);
    }
    //dataPointArray = sortDataPointArray(dataPointArray);
    return dataPointArray;
}

Note: If anything jumps out at you as conceptually flawed here, please let me know! I have spent enormous amounts of time on a fairly simple task.