Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

有人知道哪里错了么？
  public void jComboBox1_actionPerformed(ActionEvent e) {
          jComboBox3.removeAllItems();
          String item = jComboBox1.getSelectedItem().toString();
          String tableName = jList3.getSelectedValue().toString();
          String item_Eng = "";
           String tableName_Eng = "";
          try {
              Class.forName("com.mysql.jdbc.Driver");
          } catch (ClassNotFoundException e1) {
              throw new ExceptionInInitializerError(e1);
          }
          // 查询
             Connection conn = null;
             Statement st = null;
              ResultSet rs = null;
              String sql1 = "select FieldName_Eng from table_infor where FieldName_Chi = "+"'"+ item+"'" ;
              try {
            conn = JdbcUtils.getConnection();
            st = conn.createStatement();
             rs = st.executeQuery(sql1);
               while (rs.next()) {
                item_Eng = rs.getString("FieldName_Eng");
                 }
                } catch (SQLException ex) {
              } finally
                {  JdbcUtils.free(rs, st, conn);}//操作数据库完毕
         try {
             Class.forName("com.mysql.jdbc.Driver");
         } catch (ClassNotFoundException e1) {
             throw new ExceptionInInitializerError(e1);
         }
         // 查询
             String sql2 = "select TableName_Eng from tablenameengtochi where TableName_Chi = "+"'" +tableName+"'" ;
             try {
           conn = JdbcUtils.getConnection();
           st = conn.createStatement();
            rs = st.executeQuery(sql2);
              while (rs.next()) {
               tableName_Eng = rs.getString("TableName_Eng");
                }
               } catch (SQLException ex) {
             } finally
               {  JdbcUtils.free(rs, st, conn);}//操作数据库完毕               try {
             Class.forName("com.mysql.jdbc.Driver");
         } catch (ClassNotFoundException e1) {
             throw new ExceptionInInitializerError(e1);
         }
         // 查询
            String sql = "select DISTINCT " + item_Eng + " from " + tableName_Eng ;             try {
           conn = JdbcUtils.getConnection();
           st = conn.createStatement();
            rs = st.executeQuery(sql);
              while (rs.next()) {
               jComboBox3.addItem(rs.getObject(item_Eng));
                }
               } catch (SQLException ex) {
             } finally
               {  JdbcUtils.free(rs, st, conn);}//操作数据库完毕      }Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at myproject.MainFrame.jComboBox1_actionPerformed(MainFrame.java:373)
at myproject.MainFrame_jComboBox1_actionAdapter.actionPerformed(MainFrame.java:722)
at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1197)
at javax.swing.JComboBox.contentsChanged(JComboBox.java:1268)
at javax.swing.JComboBox.intervalRemoved(JComboBox.java:1288)
at javax.swing.AbstractListModel.fireIntervalRemoved(AbstractListModel.java:161)
at javax.swing.DefaultComboBoxModel.removeAllElements(DefaultComboBoxModel.java:169)
at javax.swing.JComboBox.removeAllItems(JComboBox.java:745)
at myproject.MainFrame.jList3_mouseClicked(MainFrame.java:259)
at myproject.MainFrame_jList3_mouseAdapter.mouseClicked(MainFrame.java:830)
at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:212)
at java.awt.Component.processMouseEvent(Component.java:5491)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3093)
at java.awt.Component.processEvent(Component.java:5253)
at java.awt.Container.processEvent(Container.java:1966)
at java.awt.Component.dispatchEventImpl(Component.java:3955)
at java.awt.Container.dispatchEventImpl(Container.java:2024)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4212)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3901)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3822)
at java.awt.Container.dispatchEventImpl(Container.java:2010)
at java.awt.Window.dispatchEventImpl(Window.java:1774)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)