QGraphicsView

[tbscope]

Replace

graphicsView.show();
 
MainWindow w;

graphicsView.show();

MainWindow w;

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with

MainWindow w;
 
w.setCentralWidget(&graphicsView);

MainWindow w;

w.setCentralWidget(&graphicsView);

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You created two distinct widgets and showed them.
You need to add the graphics view to the main window.