problem with qobject_cast<QWidget *>(object) (from QML object)

[Mrdata]

qmlView = new QDeclarativeView;
qmlView->setSource( QUrl::fromLocalFile( "modaldialog.qml" ) );
QObject *object = qmlView->rootObject();
 
QMetaObject::invokeMethod( object, "dialog_type_02",
					Q_RETURN_ARG( QVariant, returnedValue ),
					Q_ARG( QVariant, dlg_text )
                                         );
 
QWidget *widget = qobject_cast<QWidget *>(object);//doesn't work, return null
layout->addWidget( widget );//doesn't work, above is null
 
//however if i replace the above 2 lines of code with this:
QGraphicsObject* graphicsObject =qobject_cast<QGraphicsObject *>(object); //works fine 
scene.addItem( graphicsObject ); //works fine

qmlView = new QDeclarativeView;
qmlView->setSource( QUrl::fromLocalFile( "modaldialog.qml" ) );
QObject *object = qmlView->rootObject();

QMetaObject::invokeMethod( object, "dialog_type_02",
					Q_RETURN_ARG( QVariant, returnedValue ),
					Q_ARG( QVariant, dlg_text )
                                         );

QWidget *widget = qobject_cast<QWidget *>(object);//doesn't work, return null
layout->addWidget( widget );//doesn't work, above is null

//however if i replace the above 2 lines of code with this:
QGraphicsObject* graphicsObject =qobject_cast<QGraphicsObject *>(object); //works fine 
scene.addItem( graphicsObject ); //works fine

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why qobject_cast works ok for a QGraphicsObject and not for a QWidget coming from QML?

Why use the "layout->addWidget(...)" option? because i want a QDialog with the stuff coming from QML