问连接“qt_sql_default_connection”仍在使用中，所有查询都将停止工作

EN

public:
QSqlDatabase mydb;
void connClose()
{

    //QString connection;
    //connection = mydb.connectionName();
    mydb.close();
    //mydb.removeDatabase(connection);
    mydb.removeDatabase(mydb.connectionName());
}
bool connOpen()
{
    mydb=QSqlDatabase::addDatabase("QSQLITE");
    mydb.setDatabaseName("./Poem.db");
    if(mydb.open())
    {
        return true;
    }
    else if(!mydb.open())
    {
        return false;
    }

}

QString Title,Group,Poem;

Title = ui->lineEdit->text();
Group = ui->label_2->text();
Poem = ui->textEdit->toPlainText();



MainWindow mainwindow;
mainwindow.connOpen();



 QSqlQuery * qry1 = new QSqlQuery(mainwindow.mydb);

 qry1->prepare("insert into Poems(Title,Poem,Group) values ('"+Title+"','"+Poem+"','"+Group+"')");

 if(qry1->exec())
 {
     QMessageBox::critical(this,tr("درج شعر جدید"),tr("شعر اضافه شد"));
     mainwindow.connClose();
 }