No such slot when connecting widget signal with parent widget slot

I have the following classes:

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QStringList pluginsToStart, QWidget *parent = 0);
    ~MainWindow();

// some other stuff

public slots:
    void on_timeDataChanged(logging::TimeValueVector<bool>& aData);
    void on_importStarted();
}

and

class DataImporterWidget : public PluginWidget
{
    Q_OBJECT

public:
    explicit DataImporterWidget(QWidget *parent = 0);
    ~DataImporterWidget();

    void initConnections(QMap<QString, PluginWidget*> pluginWidgetMap);

in the method initConnections, I want the widget to init the signal-slot connections like so:

void DataImporterWidget::initConnections(QMap<QString, PluginWidget*> pluginWidgetMap)
{
    for(Importer* importer : this->getImporterMap().values())
    {
        connect(importer, SIGNAL(signal_timeDataChanged(logging::TimeValueVector<bool>&)),
            parentWidget(), SLOT(on_timeDataChanged(logging::TimeValueVector<bool>&)));
    }

    connect(this, SIGNAL(signal_importStarted()), parentWidget(), SLOT(on_importStarted()));
}

Importer is a QGroupBox and a base class for derived sub classes specifying concrete data importer types. It works like so: If I press a button, an DataImporterWidget is created and added to a QMdiArea as a QMdiSubWindow. When creating the DataImporterWidget I call the initConnections() method which sets up the signal-slot connections.

Now, when I run the program, I get the following message:

QObject::connect: No such slot 
QMdiSubWindow::on_timeDataChanged(logging::TimeValueVector<bool>&) in src/dataimporter/DataImporterWidget.cpp:81
QObject::connect: No such slot QMdiSubWindow::on_importStarted() in src/dataimporter/DataImporterWidget.cpp:85
QObject::connect:  (sender name:   'DataImporterWidget')

I do not understand why I get it because the slot is there. Even if I cast the parentWidget to the MainWindow, I get the same error.

PluginWidget is just a base class deriving from QWidget that holds some common functionality for my used plugins. I put Q_OBJECT on each base and derived class but still get this error. However, if I set up the connections in the MainWindow, it works just fine, but I wonder why the above solution won't work.

I've found the problem. The reason is, that the MainWidget class holds a QMdiArea where I add my PluginWidgets. So, when I create the PluginWidget, I set the MainWidget as its parent, but as soon as I add it to the QMdiArea, it also becomes a child of QMdiSubWindow. The parentWidget was never null but it was the wrong one ...