Qt Creator - Main widow and second window

[Kuushie118]

but sir, is there a way to fix my error?
My professor wants me to do it that way

you say that my error is that I don't have a class dialog.
but don't I have one at secwindow.h line 23??

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Added after 59 minutes:

I edited the void MainWindow::show();

void MainWindow::show()
{
    QDialog *dialog = new QDialog(this);
    connect(dialog, SIGNAL(accepted()), this, SLOT(show()));
    connect(dialog, SIGNAL(rejected()), this, SLOT(show()));
    dialog->show();
    hide();
};

void MainWindow::show()
{
    QDialog *dialog = new QDialog(this);
    connect(dialog, SIGNAL(accepted()), this, SLOT(show()));
    connect(dialog, SIGNAL(rejected()), this, SLOT(show()));
    dialog->show();
    hide();
};

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there's no error anymore but it just doesn't show my main window.

it says that:
QObject::connect: (sender name: 'MainWindow')
QObject::connect: (receiver name: 'MainWindow')


It just shows like a blank window first, then when I close that blank window...my main window will appear(in this part, the modal approach still works)
how can I fix this please?