I am running out of ideas with my problem. I have a simple gui that creates a simple test form, on that test form I want to press a button and change a label on the MainWindow. Please look at my code:

Testform.cpp

#include "testform.h"
#include "ui_testform.h"

TestForm::TestForm(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::TestForm)
{
    ui->setupUi(this);
    this->mw = new MainWindow();
    connect(ui->pB_ChangeLabelOnMainForm, &QPushButton::clicked, this->mw, &MainWindow::changeLabelOnUi);
}

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

Testform.h

#ifndef TESTFORM_H
#define TESTFORM_H

#include "mainwindow.h"
#include <QWidget>

namespace Ui {
class TestForm;
}

class TestForm : public QWidget
{
    Q_OBJECT

public:
    explicit TestForm(QWidget *parent = nullptr);
    ~TestForm();

private:
    Ui::TestForm *ui;
    MainWindow *mw;

};

#endif // TESTFORM_H

mainwindow.cpp

#include "mainwindow.h"
#include "testform.h"
#include "ui_mainwindow.h"

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

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

void MainWindow::on_pB_OpenDialog_clicked()
{
    TestForm *tf = new TestForm();
    tf->show();
}

//+++ HERE IS MY PROBLEM+++
void MainWindow::changeLabelOnUi()
{
    this->ui->lblToChangeOnMF->setText("Hello"); // NOT WORKING
    qDebug() << "TESTOUTPUT"; // WORKING
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void on_pB_OpenDialog_clicked();

public slots:
    void changeLabelOnUi();

private:
    Ui::MainWindow *ui;

};
#endif // MAINWINDOW_H

Can someone tell me why the label-change-code in the function changeLabelOnUi() in the mainwindow.cpp is not working but the qDebug() line does? When I call this function for example in the constructor, the label has changed.

I have tried another example in that the MainForm is the sender and I have changed a label on the TestForm, here was everything fine.

Thank you! :-)