Qt not removing item from layout - c++

Question

I have a simple Qt application that dynamically adds and deletes items from the layout, but it does not work, so I wrote the code below to illustrate what I'm doing:
The header file justs put an additional layout field in addition to Qt creator template:
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QFrame>
#include <QVBoxLayout>>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
private:
Ui::MainWindow *ui;
QFrame* create_frame();
void remove_frame();
QVBoxLayout* layout;
};
#endif // MAINWINDOW_H
The cpp file adds a few QFrame to the current layout, then clicking the button will remove them one by one. The QCoreApplication::processEvents and QThread::sleep just adds some animation to show what is happening.
mainwindow.cpp
#include "mainwindow.h"
#include "./ui_mainwindow.h"
#include <QPushButton>
#include <QThread>
#include <QVBoxLayout>
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
auto window = new QWidget(this);
setCentralWidget(window);
auto button = new QPushButton(window);
button->setGeometry(100, 100, 100, 100);
layout = new QVBoxLayout;
window->setLayout(layout);
for (int i = 0; i < 5; i++) {
auto x = create_frame();
layout->addWidget(x);
}
connect(button, &QPushButton::clicked, this, &MainWindow::remove_frame);
}
void MainWindow::remove_frame() {
while(layout->count() > 0) {
auto item = layout->itemAt(0);
layout->removeItem(item);
delete item;
QCoreApplication::processEvents();
QThread::msleep(200);
}
}
QFrame* MainWindow::create_frame() {
auto x = new QFrame();
x->setMinimumHeight(20);
x->setMaximumHeight(20);
x->setFrameStyle(QFrame::StyledPanel);
return x;
}
MainWindow::~MainWindow()
{
delete ui;
}
The animation is a mess, and does not completely remove all the QFrame boxes. What did I do wrong here? And later I find that if I use auto item = layout->itemAt(0)->widget(); layout->removeWidget(item); then I can indeed remove them, are removeItem and removeWidget doing different things?