When using JOption Pane, During Cancelled JOption Pane, NullPointerException occurs

I have data in jTable1. I attempted to send them to jTable2 , after calculating by value entered to JOption Pane. But when I clicked on cancelled button on JOption Pane program making error. Line 80 in my code is

BigDecimal quantity = new BigDecimal(JOptionPane.showInputDialog(null,"Enter Quantity", 0));

I need help to solve this problem. Thanks in advance.

Code is shown below

    import java.math.BigDecimal;
    import javax.swing.JOptionPane;
    import javax.swing.table.DefaultTableModel;

    public class TableGUI extends javax.swing.JFrame {

    public TableGUI() {
        initComponents();
    }

    private void initComponents() {
        java.awt.GridBagConstraints gridBagConstraints;

        jScrollPane1 = new javax.swing.JScrollPane();
        jTable1 = new javax.swing.JTable();
        jScrollPane2 = new javax.swing.JScrollPane();
        jTable2 = new javax.swing.JTable();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        getContentPane().setLayout(new java.awt.GridBagLayout());

        jTable1.setModel(new javax.swing.table.DefaultTableModel(
                new Object[][]{
                    {"1", "Cement", "cwt", "1050.00", "1.00", "2.00", "2100.00"},
                    {"2", "Sand", "cube", "7500.00", "1.00", "1.50", "11250.00"},
                    {"3", "Skill labour", "Day", "2500.00", "1.00", "2.50", "6250.00"},
                    {"4", "Unskill labour", "Day", "1500.00", "1.00", "2.50", "3750.00"}
                },
                new String[]{
                    "ID", "Name", "Unit", "Rate", "Usage", "Quantity", "Amount"
                }
        ));
        jTable1.setRowHeight(25);
        jScrollPane1.setViewportView(jTable1);

        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.fill = java.awt.GridBagConstraints.BOTH;
        gridBagConstraints.weightx = 0.1;
        gridBagConstraints.weighty = 0.1;
        gridBagConstraints.insets = new java.awt.Insets(5, 5, 5, 5);
        getContentPane().add(jScrollPane1, gridBagConstraints);

        jTable2.setModel(new javax.swing.table.DefaultTableModel(
                new Object[][]{},
                new String[]{
                    "ID", "Name", "Unit", "Rate", "Usage", "Quantity", "Amount"
                }
        ));
        jTable2.setRowHeight(25);
        jScrollPane2.setViewportView(jTable2);

        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.fill = java.awt.GridBagConstraints.BOTH;
        gridBagConstraints.weightx = 0.1;
        gridBagConstraints.weighty = 0.1;
        gridBagConstraints.insets = new java.awt.Insets(5, 5, 5, 5);
        getContentPane().add(jScrollPane2, gridBagConstraints);

        jButton1.setText("Calculate");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 1;
        gridBagConstraints.gridwidth = 2;
        gridBagConstraints.insets = new java.awt.Insets(5, 5, 5, 5);
        getContentPane().add(jButton1, gridBagConstraints);

        pack();
    }

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {

        BigDecimal quantity = new BigDecimal(JOptionPane.showInputDialog(null,"Enter Quantity", 
     0));

        if (quantity != null && quantity.equals("")) {
            
            JOptionPane.showMessageDialog(null, "Calculation Cancelled");
            
        } else {
            DefaultTableModel model1 = (DefaultTableModel) jTable1.getModel();

            for (int i = 0; i < model1.getRowCount(); i++) {

    Object data1 = jTable1.getValueAt(i, 0);
    Object data2 = jTable1.getValueAt(i, 1);
    Object data3 = jTable1.getValueAt(i, 2);
    BigDecimal data4 = new BigDecimal((String) jTable1.getValueAt(i, 3).toString());
    BigDecimal data5 = new BigDecimal((String) jTable1.getValueAt(i, 4).toString());
    BigDecimal data6 = quantity.multiply(new BigDecimal((String) 
    jTable1.getValueAt(i,5).toString()));
    data6 = data6.setScale(2, BigDecimal.ROUND_HALF_UP);
    BigDecimal data7 = data4.multiply(data5.multiply(data6));
    data7 = data7.setScale(2, BigDecimal.ROUND_HALF_UP);

                Object[] row = {data1, data2, data3, data4, data5, data6, data7};
                DefaultTableModel model2 = (DefaultTableModel) jTable2.getModel();
                model2.addRow(row);
            }
        }
    }

    public static void main(String args[]) {

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new TableGUI().setVisible(true);
            }
        });
    }

    private javax.swing.JButton jButton1;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JScrollPane jScrollPane2;
    private javax.swing.JTable jTable1;
    private javax.swing.JTable jTable2;

   }

Stack Trace shown below

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at java.math.BigDecimal.<init>(BigDecimal.java:806)
at soquestion.TableGUI.jButton1ActionPerformed(TableGUI.java:80)
at soquestion.TableGUI.access$000(TableGUI.java:7)
at soquestion.TableGUI$1.actionPerformed(TableGUI.java:65)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6535)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6300)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4891)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer

I changed my code according to the given opinions. It works well now. Thnaks all who helped me. @QBrute,@Pshemo, @ControlAltDel

  String userValue = JOptionPane.showInputDialog("Enter Quantity");

    if (userValue != null && !userValue.isEmpty()) {

        BigDecimal quantity = new BigDecimal(userValue);

        DefaultTableModel model1 = (DefaultTableModel) jTable1.getModel();

        for (int i = 0; i < model1.getRowCount(); i++) {

            Object data1 = jTable1.getValueAt(i, 0);
            Object data2 = jTable1.getValueAt(i, 1);
            Object data3 = jTable1.getValueAt(i, 2);
            BigDecimal data4 = new BigDecimal((String) jTable1.getValueAt(i, 3).toString());
            BigDecimal data5 = new BigDecimal((String) jTable1.getValueAt(i, 4).toString());
            BigDecimal data6 = quantity.multiply(new BigDecimal((String) jTable1.getValueAt(i, 5).toString()));
            data6 = data6.setScale(2, BigDecimal.ROUND_HALF_UP);
            BigDecimal data7 = data4.multiply(data5.multiply(data6));
            data7 = data7.setScale(2, BigDecimal.ROUND_HALF_UP);

            Object[] row = {data1, data2, data3, data4, data5, data6, data7};
            DefaultTableModel model2 = (DefaultTableModel) jTable2.getModel();
            model2.addRow(row);
        }

    } else {
        JOptionPane.showMessageDialog(null, "Calculation Cancelled");
    }