Java:线程异常“AWT-EngEngQueE-0”JavaLang.Null PoExExtExchange

public class MainFrame extends JFrame {
    public DefaultTableModel historyModel;
    public DefaultTableModel dataModel;
    public JTable historyTable;
    public JTable dataTable;
    public JTableHeader header;

private void createUI() {
        browsersTabbedPane = new BrowsersTabbedPane();
        add(browsersTabbedPane.getTabPane(), CENTER);
        addDefaultBrowserComponentForTabbedPane();
        createActions();
        add(createToolBar(), BorderLayout.NORTH);
        add(createStatusBar(), BorderLayout.SOUTH);
        add(createDataView(dataModel,dataTable, header), BorderLayout.PAGE_END);
        add(createHistoryView(historyModel,historyTable), BorderLayout.LINE_END);
        add(createBigButton(browsersTabbedPane.getActiveBrowser(),dataModel,historyModel,dataTable,historyTable), BorderLayout.WEST);

        createMenuBar();
    }

private JPanel createDataView(DefaultTableModel dataModel, JTable dataTable, JTableHeader header){
        JPanel panel = new JPanel();

        Object data[][] = {{"001","vinod","Bihar","India","Biology","65","First"},
                           {"002","Raju","ABC","Kanada","Geography","58","second"},
                           {"003","Aman","Delhi","India","computer","98","Dictontion"},
                           {"004","Ranjan","Bangloor","India","chemestry","90","Dictontion"},
                           {"004","Ranjan","Bangloor","India","chemestry","90","Dictontion"}};
        Object col[] = {"Roll","Name","State","country","Math","Marks","Grade"};
        dataModel = new DefaultTableModel(data,col);
        dataTable = new JTable(dataModel);

        dataTable.setPreferredScrollableViewportSize(new Dimension(1500,100));
        header = dataTable.getTableHeader();
        header.setBackground(Color.yellow);
        JScrollPane pane = new JScrollPane(dataTable);
        dataTable.setAutoResizeMode(JTable.WIDTH);
        dataTable.doLayout();
        panel.add(pane);
        return panel;

    }

    private JPanel createBigButton(Browser browser, DefaultTableModel dataModel, DefaultTableModel historyModel, JTable dataTable, JTable historyTable){
        JPanel panel = new JPanel();
        JButton bigbutton = new JButton("Big Button");
        bigbutton.addActionListener(new BigButtonListener(browser,dataModel,historyModel,dataTable, historyTable));
        panel.add(bigbutton);
        return panel;
    }

   class BigButtonListener implements ActionListener{
        Image screenshot;
        Browser bigbrowser;

        BigButtonListener(Browser browser, DefaultTableModel dataModel, DefaultTableModel historyModel, JTable dataTable, JTable historyTable) {
            screenshot = browser.toImage(true);
            bigbrowser = browser;
            historyModel = historyModel;
            dataModel = dataModel;
            dataTable = dataTable;
            historyTable = historyTable;
            //table1.addRow
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            // save current image           
            //historyModel.insertRow(2,new Object[]{"Sushil"}); 
            System.out.println(historyTable.getRowCount());

        }   
    }  
}

单击大按钮时出现问题。它应该输出HistoryTable(公共)的总行数。

但是,这是stacktrace输出

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.my.demo.MainFrame$BigButtonListener.actionPerformed(MainFrame.java:356)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)