Qt app (with while loop) still running after closing MainWindow - loops

Question

I have this sample code that reproduces the problem I am dealing with.
//main.cpp
#include "widget.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
widget w;
w.show();
return a.exec();
}
--
//widget.h
#pragma once
#include <QMainWindow>
#include <QPushButton>
#include <QApplication>
#include <QLayout>
class widget : public QMainWindow
{
Q_OBJECT
public:
QApplication* application;
widget(QWidget *parent = nullptr);
QVBoxLayout *mainLayout;
QPushButton *startButton;
QWidget *centralWidget;
~widget();
public slots:
void onStartButtonPressed();
private:
void start();
bool loop;
};
--
//widget.cpp
#include "widget.h"
widget::widget(QWidget *parent)
: QMainWindow(parent)
{
application = static_cast<QApplication*>(QApplication::instance());
mainLayout = new QVBoxLayout();
loop = false;
startButton = new QPushButton("START");
mainLayout->addWidget(startButton);
connect(startButton, SIGNAL(clicked()),this,SLOT(onStartButtonPressed()));
centralWidget = new QWidget();
centralWidget->setLayout(mainLayout);
this->setCentralWidget(centralWidget);
this->showMaximized();
this->setWindowTitle("Widget");
}
void widget::onStartButtonPressed()
{
loop = !loop;
startButton->setText(loop? "STOP" : "START");
start();
}
void widget::start()
{
while(loop)
{
application->processEvents();
}
}
widget::~widget()
{
loop = false;
}
This is a simple Qt application with a QPushButton that when pressed set a boolean flag to true and starts a loop. When the main widget is closed the destructor sets boolean flag to false and the loop should break.
However when loop is running the destructor seems not been called, while when loop is not running it is called normally and the app quit accordingly.
Of course I call application->processEvents() to prevent freezing, but I don't know why the destructor is not called at loop running.