"AWT-EventQueue-0" java.lang.NullPointerException - need some advices

i'm new in Java World. I'm trying to create a simple app, populating a JTable from a local database. I have some issues with this error (shown below) and I can't realize what's going on...

Error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at ProiectDealerAuto.SwingInterface.Button4ActionPerformed(SwingInterface.java:231)
at ProiectDealerAuto.SwingInterface.access$200(SwingInterface.java:22)
at ProiectDealerAuto.SwingInterface$3.actionPerformed(SwingInterface.java:97)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
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:6527)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6292)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4883)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2739)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
at java.awt.EventQueue.access$400(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:697)
at java.awt.EventQueue$3.run(EventQueue.java:691)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:719)
at java.awt.EventQueue$4.run(EventQueue.java:717)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
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)

I want to populate JTable when i press a button. This is the try-catch block for my button.

Connection connection = null;
    Statement statement = null;


    try{

     Class.forName("com.mysql.jdbc.Driver");
     connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/dealerauto","cosmin","kilimanjaro");
        ResultSet result = statement.executeQuery("SELECT * FROM cars");
        ResultSetMetaData rsmetadata = result.getMetaData();
        int coloumns = rsmetadata.getColumnCount();
        DefaultTableModel dtm = new DefaultTableModel();
        Vector coloumns_name = new Vector();
        Vector data_rows = new Vector();

        for (int i=1; i < coloumns; i++)
        {
            coloumns_name.addElement(rsmetadata.getColumnName(i));
        }
        dtm.setColumnIdentifiers(coloumns_name);
        while(result.next())
        {
            data_rows = new Vector();
            for(int j=1; j<coloumns; j++)
            {
                data_rows.addElement(result.getString(j));
           }
            dtm.addRow(data_rows);
        }

        tbCars.setModel(dtm);


    }
      catch(ClassNotFoundException error)
   {
     System.out.println("Error "+ error.getMessage());
   }
    catch(SQLException error){
        System.out.println("Error "+ error.getMessage());
    }