getting a null pointer, object is most definatly instantiated

Below is the code for my class:

package orthopedicnotegenerator;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.font.*;
import java.text.Collator;

class DxPrEdit extends JDialog implements ActionListener {

    DefaultListModel diagnosisListModel;
    String diagnosisIndex[][];
    JList diagnosisList;
    JScrollPane diagnosisScroller;
    String sSN;

    DataConnection dataConnection = new DataConnection();

    public DxPrEdit(String stringSSN, String selectedDiagnosisInput) {

        sSN = stringSSN;

        setModal(true);
        Container pane = this.getContentPane();
        this.setAlwaysOnTop(true);
        pane.setLayout(null);
        addWidgets();

        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        setBounds(300,100,700,500);

        populateDxList(selectedDiagnosisInput);

        setVisible(true);
        validate();

        diagnosisList.setSelectedValue(selectedDiagnosisInput, true);
    }

    public void addWidgets(){

        diagnosisListModel = new DefaultListModel();
        JList diagnosisList = new JList(diagnosisListModel);
        diagnosisScroller = new JScrollPane(diagnosisList);

        JLabel dxHeader = new JLabel("Diagnoses");
        dxHeader.setFont(new Font("Serif", Font.BOLD, 30));
        add(dxHeader);
        dxHeader.setBounds(10,10, dxHeader.getPreferredSize().width,
dxHeader.getPreferredSize().height);

        add(diagnosisScroller);
        diagnosisScroller.setBounds(10,60, 500, 100);

    }

    public void populateDxList(String selectedEntry){
        diagnosisListModel.removeAllElements();
        diagnosisIndex = null;

        String sql = "SELECT diagnosisID, exam_NewID,
exam_New_ProblemsID, relatedDiagnosisID, diagnosis, onsetDate, active,
" +
                "prePost, iCD FROM diagnoses WHERE sSN=\"" + sSN + "\"
ORDER BY active;";

        DefaultListModel queryResults =
dataConnection.queryConnection(sql);

        diagnosisIndex = new String[queryResults.getSize() / 9][9];

        int counter = 0;

        for(int i = 0; i < diagnosisIndex.length; i++){
            for(int j = 0; j < 9; j++){
                diagnosisIndex[i][j] =
String.valueOf(queryResults.getElementAt(counter++));
            }
        }

        Collator myCollator = Collator.getInstance();
        for(int i = 0; i < diagnosisIndex.length; i++){
            for(int j = 0; j < diagnosisIndex.length - 1; j++){
                if( myCollator.compare(diagnosisIndex[j][6],
diagnosisIndex[j + 1][6]) > 0 ){

                    String temp[] = new String[9];

                    temp = diagnosisIndex[j];
                    diagnosisIndex[j] = diagnosisIndex[j + 1];
                    diagnosisIndex[j + 1] = temp;

                }
            }
        }

        for(int i = 0; i < diagnosisIndex.length; i++){
            for(int j = 0; j < diagnosisIndex.length - 1; j++){
                if(diagnosisIndex[j][6].equals("Active") &&
diagnosisIndex[j + 1][6].equals("Active")){
                    if(
myCollator.compare(diagnosisIndex[j][5],diagnosisIndex[j + 1][5]) > 0
){

                    String temp[] = new String[9];

                    temp = diagnosisIndex[i];
                    diagnosisIndex[j] = diagnosisIndex[j + 1];
                    diagnosisIndex[j + 1] = temp;
                    }
                }

                else if(diagnosisIndex[j][6].equals("Inactive") &&
diagnosisIndex[j + 1][6].equals("Inactive")){
                    if(
myCollator.compare(diagnosisIndex[j][5],diagnosisIndex[j + 1][5]) < 0
){

                    String temp[] = new String[9];

                    temp = diagnosisIndex[i];
                    diagnosisIndex[j] = diagnosisIndex[j + 1];
                    diagnosisIndex[j + 1] = temp;
                    }
                }

        }
        }

        for(int i = 0; i < diagnosisIndex.length; i++){
            diagnosisListModel.addElement(diagnosisIndex[i][6] + " - "
+ diagnosisIndex[i][5] + " - " + diagnosisIndex[i][8] + " "
                                   + " " + diagnosisIndex[i][4] );
        }

        diagnosisScroller.validate();

    }

    public void updateDx(){

    }

    public void actionPerformed(ActionEvent ae) {

        dispose();
        }

//*********************************************************************************

//*********************************************************************************

//*********************************************************************************
    public static void main(String args[]){
        DxPrEdit dxpr = new DxPrEdit("123-45-6789", "");
    }

}

DataConnection is a class that connects to my database. It runs
queries returning the results in a DefaultListModel. The
diagnosisListModel is then populated by different elements of the
returned records. And, in turn, the JList, diagnosisList, is passed
diagnosisListModel to provide the selectable elements.

I am running into a problem when I try to set the selected object in
the JList. It is giving me a null pointer exception at the line where
I attempt to assign a value to the JList.

I also noticed, as I'm playing with it more, that the null pointer is
occuring as I close the dialog box.

As always, any and all help is greatly appreciated.