我有这个代码,我想从jcomboBox获取所选项目,但是当我运行我的项目时,它给我复制打印所选项目的值和java.Lang.NullPointerException
这是代码：

 private void jComboBox4ItemStateChanged(java.awt.event.ItemEvent evt) {                                            
        // TODO add your handling code here:
         if (evt.getStateChange()==ItemEvent.SELECTED){

             String a=String.valueOf(jComboBox4.getSelectedItem());
         System.out.print(a);

         try{
        String del2="select distinct PTYPE from Projects inner join project on projects.PNUMBER=(select pro_id from project where pro_name='"+a+"')";
         psst=con.prepareStatement(del2);
        String td2;
          DefaultComboBoxModel mode2 = new DefaultComboBoxModel();
           ResultSet RSS=psst.executeQuery();
           while(RSS.next()){
            td2=RSS.getString("PTYPE");
    mode2.addElement(td2);
       jComboBox7.setModel(mode2);
           }
    }
        catch(sqlException ex){
            JOptionPane.showMessageDialog(null,ex.toString());
 } 
}