QSqlDatabase no error printed and no output?

Question

I use QSqlDatabase to insert data to mysql.

First, I define a class named Inserter:

class Inserter
{
    QSqlDatabase db_connection;
    
public:
    Insert()
    {
        db_connection = QSqlDatabase::addDatabase("QMYSQL");

        db_connection.setDatabaseName("dbname");

        db_connection.setHostName("localhost");

        db_connection.setUserName("root");

        db_connection.setPassword("psd");

        if(!db_connection.open())
        {
            qDebug() << db_connection.lastError();
        }
    }
};

And then, I use Insert in another class defined as Scheduler:

class Scheduler
{
    Inserter inserter;
public:
    Scheduler()
    {
        inserter = Inserter();
    }
};

When I run this program, Qt print warning:

QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.

QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

someone said:

QSqlDatabase::addDatabase() is supposed to be called once per type and connection name. Calling addDatabase("QMYSQL") multiple times leads to QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

Furthermore, having existing QSqlQuery objects while removing a database leads to QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.

If the above reason of the warning is right, why the sequence of the warning is not to be:

QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.

At last, I move the initial code to the class Scheduler as follow:

class Scheduler
{
    QSqlDatabase db_connection;
  
    void initDBConnection()
    {
        db_connection = QSqlDatabase::addDatabase("QMYSQL");

        db_connection.setDatabaseName("walmart");

        db_connection.setHostName("localhost");

        db_connection.setUserName("root");

        db_connection.setPassword("");
 
        if(!db_connection.open())
        {
            qDebug() << db_connection.lastError();
        }
    }

public:
    Scheduler()
    {
        initDBConnection();
    }
}; 

There is no warning or errors printed.