Why is my variable being reset to Null?

My main class

Objective: Check a string value from another class so I can use that value to make calculations
Problem: String value is null on second click so it throws a nullpointer exception.

private void jLabel63MouseClicked(java.awt.event.MouseEvent evt) {
        randomResource rg3 = new randomResource();
        rg3.setResource(lumber63, brick63, coin63, grain63);

        System.out.println(rg3.resourceValue);
        if (!(discovered63 == true)) {
            rg3.genNum(clicked63);
        }
        if (rg3.resourceValue.equals("lumber")) {
            jLabel63.setIcon(new ImageIcon("src/hexTree.png"));
            lumber63 = true;
            discovered63 = true;
            rg3.setResource(lumber63, brick63, coin63, grain63);
            if (villagerAmount <= 0) {
                jLabel63.setIcon(new ImageIcon("src/hexTreeRedFull.png"));
            }
            if (villagerAmount >= 1) {
                jLabel63.setIcon(new ImageIcon("src/hexTreeCutting.png"));
                harvestReady = true;

                ActionListener taskPerformer5 = new ActionListener() {
                    public void actionPerformed(ActionEvent evt) {

                        while (buffRemaining >= 0) {
                            resource1Amount++;
                            buffRemaining--;
                        }
                        if (buffRemaining <= 0) {
                            harvesting = false;
                        }
                    }
                };
                new Timer(delayThreeTwo, taskPerformer5).start();

                villagerAmount--;
                villagers.setText("Villagers: " + villagerAmount);
            }
        }
        if (rg3.resourceValue.equals("bricks")) {
            jLabel63.setIcon(new ImageIcon("src/hexRock.png"));
            discovered63 = true;
            brick63 = true;
            rg3.setResource(lumber63, brick63, coin63, grain63);
        }
        if (rg3.resourceValue.equals("grain")) {
            jLabel63.setIcon(new ImageIcon("src/defaultFood.png"));
            grain63 = true;
            discovered63 = true;
            rg3.setResource(lumber63, brick63, coin63, grain63);
        }
        if (rg3.resourceValue.equals("coin")) {
            jLabel63.setIcon(new ImageIcon("src/hexCoin.png"));
            coin63 = true;
            discovered63 = true;
            rg3.setResource(lumber63, brick63, coin63, grain63);

        } else {
            System.out.println("else, your program sucks. ");
            discovered63 = true;
        }

    }

 if (rg3.resourceValue.equals("lumber")) 

It's this line that gives me a nullpointer. Or any of the the .equals lines. It works the first time, honestly I don't know why... It says the value is null, but on the second mouse click it is set to null but doesn't work and error / crashes program.

Here is the class I'm using

import java.util.Random;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
/**
 *
 * @author Matthew
 */
public class randomResource {

    PlayingField field = new PlayingField();
    public boolean chosenResource;
    public int randomResource;

    public boolean discovered;
    public boolean lumber;
    public boolean grain;
    public boolean coin;
    public boolean brick;
    public String resourceValue;

    
    public String genNum(boolean chosen) {

        resourceValue = "";
        Random rg = new Random();
        if (!(chosenResource == true)) {
            randomResource = rg.nextInt(99) + 1;
        }
        if (randomResource >= 1 && randomResource <= 25) {
            lumber = true;
            chosenResource = true;
            resourceValue += "lumber";
        }
        if (randomResource >= 25 && randomResource <= 50) {
            brick = true;
            chosenResource = true;
            resourceValue += "bricks";
        }
        if (randomResource >= 50 && randomResource <= 75) {
            grain = true;
            chosenResource = true;
            resourceValue += "grain";
        }
        if (randomResource >= 75 && randomResource <= 100) {
            coin = true;
            chosenResource = true;
            resourceValue += "coin";

        } else {
        }
        return resourceValue;
    }