Adding class to MainWindow - using QHBoxLayout ?

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My objective is to develop the application using modular approach.
Using QtDesigner I build EMPTY MainWindow , then using same method I build another blank, EMPTY class.
In this example QMdiArea.
Next step is to populate QMdiArea class / form as desired.

Next step is to ADD the QMdiArea class to MainWindow - so it became itergral part of MainWindow, no loger idependelty floating.

I found this snippet of code which supposedly works, but I am getting runtime error
and my QMdiArea widget is stiil afloat, pretty much out of control of main window.

// create layout
    // Set layout
    QHBoxLayout *layout = new QHBoxLayout;
    this->setLayout(layout);
    // add  mdi area class 
    MdiArea *mdiarea = new MdiArea();
    mdiarea->show();
    // add to main window
    layout->addWidget(mdiarea);
    mdiarea->show();

// create layout
    // Set layout
    QHBoxLayout *layout = new QHBoxLayout;
    this->setLayout(layout);
    // add  mdi area class 
    MdiArea *mdiarea = new MdiArea();
    mdiarea->show();
    // add to main window
    layout->addWidget(mdiarea);
    mdiarea->show();

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Here is the error

Starting /media/z/DEV_COPY_LABEL/Qt/QT/qtconnectivity/examples/bluetooth/build-CAT_BT-Desktop-Debug/btscanner...
QWidget::setLayout: Attempting to set QLayout "" on MainWindow_BT_MDI "MainWindow_BT_MDI", which already has a layout

OK, main window has "layout" - so how do I physically use it to add my QMdiArea class ?

PS
There seems to be standard method to add "QDock" to main window , but I have not tried it, not yet - next task.