Add MarbleWidget into main window on QT

I'm trying to integrate a MarbleWidget into a QT project. If I only want to show the widget like this:

Marble::MarbleWidget *mapWidget = new Marble::MarbleWidget();
mapWidget->setMapThemeId("earth/openstreetmap/openstreetmap.dgml");
mapWidget->show()

everything is fine. But what I would like to do is integrate the widget into an existing window. What I did is that I added a widget using QtDesigner, promoted the widget to Marble::MarbleWidget then used the following code:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->widget = new Marble::MarbleWidget();
    ui->widget->setMapThemeId("earth/openstreetmap/openstreetmap.dgml");
    ui->setupUi(this);
}

I can see the widget but no map:

and I see the following errors on the console:

QObject::connect: No such signal MarbleWebView::titleChanged(QString) in ~/marble/sources/src/lib/marble/PopupItem.cpp:71
QObject::connect:  (sender name:   'webView')
QObject::connect:  (receiver name: 'titleText')
QObject::connect: No such signal MarbleWebView::urlChanged(QUrl) in ~/marble/sources/src/lib/marble/PopupItem.cpp:72
QObject::connect:  (sender name:   'webView')
QObject::connect: No such signal MarbleWebView::titleChanged(QString) in ~/marble/sources/src/lib/marble/PopupItem.cpp:71
QObject::connect:  (sender name:   'webView')
QObject::connect:  (receiver name: 'titleText')
QObject::connect: No such signal MarbleWebView::urlChanged(QUrl) in ~/marble/sources/src/lib/marble/PopupItem.cpp:72
QObject::connect:  (sender name:   'webView')

What did I do wrong?

What Kevin said. Code-wise that would be

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->widget->setMapThemeId("earth/openstreetmap/openstreetmap.dgml");
}