Why doesn't the QQuickWidget program display when using QQmlApplicationEngine replacement？

[mirro187187]

I modify the program support QT5.8.

` QQucikWidget* pWidget = new QQucikWidget();
pWidget->setResizeMode(QuickWidget:SizeRootObjectToView);

QStandardItemModel standardItemModel;
QStandardItem root("Root");
root.setIcon(qtApp.style()->standardIcon(QStyle::SP_DesktopIcon));
QStandardItem child1("Child 1");
QStandardItem child2("Child 2");
QStandardItem child3("Child 3");
QStandardItem child1OfChild1("Child 1 of Child 1");
QStandardItem child2OfChild1("Child 2 of Child 1");
QStandardItem child1OfChild2("Child 1 of Child 2");

child1.appendRow(&child1OfChild1);
child1.appendRow(&child2OfChild1);
child2.appendRow(&child1OfChild2);
root.appendRow(&child1);
root.appendRow(&child2);
root.appendRow(&child3);
standardItemModel.appendRow(&root);

TreeViewModel standardItemTreeViewModel;
standardItemTreeViewModel.setSourceModel(&standardItemModel);

pWidget ->rootContext()->setContextProperty("standardItemModel", &standardItemTreeViewModel);

 pWidget ->load(QUrl("qrc:/main.qml"));`