QSqlDatabase strange behavior

Hi everyone,
I am trying mysql database connection with QSqlDatabase and it's behaving really strange. When I have only one button in the GUI, everything works fine. But when I add Line Edit, which doesn't do anything, program hangs for about 4 seconds on exit.
Only thing the program does is that it creates connection to database in constructor and then closes it in destructor.

Here is the code:

Code:

---

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    db = QSqlDatabase::addDatabase("QMYSQL", "connection");
    db.setHostName("127.0.0.1");
    db.setDatabaseName("db");
    db.setUserName("user");
    db.setPassword("password");
    connected = db.open();
}
 
MainWindow::~MainWindow()
{
    db.close();
    db = QSqlDatabase();
    QSqlDatabase::removeDatabase("connection");
    delete ui;
}

---

If I put this code in "on_pushButton_clicked()" function, program exits correctly. But is it okay to create and remove connection to database in every function of the program, when I want to create more complex database solution?

Another strange thing is that I had to add "db = QSqlDatabase();" before I call removeDatabase, because it showed warning about connection being used when program exited.

Is someone here who can explain it to me, please? :)

I forgot to mention that this issue is on windows, it is working properly on linux. So maybe some bug?