QSqlDatabasePrivate：：addDatabase： 重复的连接名称'MyConnection'错误

我已经在C Inorder中编写了QT程序，以访问数据库并加载数据库表来表达表视图！我的程序没有显示任何编译错误，但给出了2个运行时错误

QSqlDatabasePrivate::removeDatabase: connection 'MyConnection' is still in use, all queries will cease to work.
QSqlDatabasePrivate::addDatabase: duplicate connection name 'MyConnection', old connection removed.

主要有MainWindow的类（具有数据库连接方法）和对话框以下是我的代码

mainWindow.h

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
public: QSqlDatabase mydb;
        bool conOpen(QString userName,QString password,QString hostname,int port,QString service){
            mydb=QSqlDatabase::addDatabase("QOCI","MyConnection");
            mydb.setUserName(userName);
            mydb.setPassword(password);
            mydb.setHostName(hostname);
            mydb.setPort(port);
            mydb.setDatabaseName(service);
            return mydb.open();
}

mainwindow.cpp

void MainWindow::on_pushButton_clicked()
{
    Dialog *dialog1 = new Dialog(this);
   if(conOpen(ui->uname->text(),ui->pword->text(),ui->ip->text(),ui->port->text().toInt(),ui->service->text())){
        dialog1->show();
}

dialog.h

class Dialog : public QDialog
{
    Q_OBJECT
public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();
private slots:
    void on_pushButton_clicked();

dialog.cpp

void Dialog::on_pushButton_clicked()
{
    QSqlQueryModel *modal = new QSqlQueryModel();
    if(QSqlDatabase::contains("MyConnection")){
        QSqlQuery* qry=new QSqlQuery(QSqlDatabase::database("MyConnection"));
        qry->prepare("select NAME FROM Employees");
           qry->exec();
        modal->setQuery(*qry);
      ui->tableView->setModel(modal);
    }
}

MainWindow表单用于登录，对话框用于将数据库表中的条目检索到表视图我该如何纠正问题？