MySql: my_thread_global_end(): XX threads didn't exit

#include <QtCore>
#include <QtSql>

class Client: public QRunnable
{
public:
    virtual void run();
};

void Client::run()
{
    {
        QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", QString::number((unsigned long long)QThread::currentThreadId()));

        db.setHostName("localhost");
        db.setDatabaseName("test");
        db.setUserName("me");
        db.setPassword("mypass");

        db.open();
        
        db.close();
    }

    QSqlDatabase::removeDatabase(QString::number((unsigned long long)QThread::currentThreadId()));

}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QThreadPool pool;
    pool.setMaxThreadCount(100);

    for (int i = 0; i < 100; ++i)
    {
        Client *client = new Client;
        bool started = pool.tryStart(client);
        Q_ASSERT(started);
    }

    pool.waitForDone();

    //return a.exec();
    return 0;
}