Quit the application in Qt

I created an application in Qt that contains two buttons: an exit button and an import button. When the import button is pressed, a list of buttons is displayed on the scroll screen (the loggers.csv file contains data 1; 2; 3; 4; 5;).

Everything works fine, but when I press the exit button (which, of course, should close everything), the application does not stop properly (the Qt stop button is still active, but the play button is not). When I start the debugger and press the exit button, I get an error message: Invalid address specified for RtlFreeHeap (0ADF0000, 0028FE40). Can anybody help me?

Main

#include <QtGui/QApplication>
#include "mainwindow.h"

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

    MainWindow w;

    w.showFullScreen();

    return a.exec();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QtGui>
#include "logger.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    QPushButton exit_btn;
    QPushButton import_btn;

private slots:

    void createMenus();
    void exit();
    void import();

private:

    int window_width;
    int window_height;

    int numLoggers;
    int numSelected;

    QVector<Logger*> loggers;

    QScrollArea * scroll_area;

    QVBoxLayout scrollLayout;

    QWidget viewport;

    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "QtGui"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    window_width = QApplication::desktop()->width();
    window_height = QApplication::desktop()->height();

    createMenus();

    connect(&exit_btn,SIGNAL(clicked()),this,SLOT(exit()));
    connect(&import_btn,SIGNAL(clicked()),this,SLOT(import()));
}

MainWindow::~MainWindow()
{
   delete ui;
}

void MainWindow::createMenus()
{
    import_btn.setParent(ui->centralWidget);
    import_btn.setGeometry(400,300,100,100);
    import_btn.setText("IMPORT");

    exit_btn.setText("EXIT");
    exit_btn.setParent(ui->centralWidget);
    exit_btn.setGeometry(window_width-50,12,32,32);

    viewport.setLayout(&scrollLayout);
    viewport.resize(0,0);

    scroll_area = new QScrollArea(ui->centralWidget);
    scroll_area->setGeometry(0,66,317,window_height-116);
    scroll_area->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    scroll_area->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    scroll_area->setWidget(&viewport);
    scroll_area->setGeometry(0,97,317,window_height-228);

    scrollLayout.setMargin(0);
    scrollLayout.setSpacing(0);
}

void MainWindow::exit()
{
    close();
    qApp->quit();
}

void MainWindow::import()
{
    numSelected=0;

    QFile f("Loggers3.csv");

    if (f.open(QIODevice::ReadOnly))
    {
        numLoggers=0;

        QString data;
        data = f.readAll();
        QStringList vals = data.split(';');

        while(vals.size()>=1)
        {
            Logger * logger = new Logger;

            logger->setNumber(vals[0].toInt());
            vals.removeAt(0);

            loggers<<logger;

            numLoggers++;
        }
        f.close();


        for(int i=0; i<numLoggers;i++)
        {
            loggers[i]->createButtons();
            scrollLayout.addWidget(loggers[i]->button);
        }

        viewport.resize(367,numLoggers*60);
    }
}

logger.h

#ifndef LOGGER_H
#define LOGGER_H

#include <QtGui>

class Logger : public QWidget
{
    Q_OBJECT
public:
    explicit Logger(QWidget *parent = 0);

    ~Logger();
    int number;
    QLabel num;
    QToolButton * button;
    bool checked;

signals:

public slots:

    void setNumber(int number);
    void createButtons();
};

#endif // LOGGER_H

logger.cpp

#include "logger.h"
#include <QtGui>

Logger::Logger(QWidget *parent) :
    QWidget(parent)
{
    button = new QToolButton;
    button->setCheckable(true);
    button->setMinimumSize(317,60);
    button->setStyleSheet("QToolButton{background-image: url(images/btn_bg); border:none}");
}

Logger::~Logger()
{
}

void Logger::setNumber(int logNumber)
{
    number=logNumber;
}

void Logger::createButtons()
{
    QLayout * layout = new QHBoxLayout;

    QSpacerItem *spacer = new QSpacerItem(120, 31, QSizePolicy::Maximum, SizePolicy::Maximum);

    num.setStyleSheet("color: white; font: bold 16px");
    num.setText(QString::number(number));

    layout->addWidget(&num);
    layout->addItem(spacer);

    button->setLayout(layout);
}