Use qGuiApp instead of qApp

As QtQuick applications might not have QApplication.

src/qtquick/views/Stack_qtquick.cpp

@@ -208,6 +208,7 @@ void DockWidgetModel::emitDataChangedFor(Controllers::DockWidget *dw)
 
 void DockWidgetModel::remove(Controllers::DockWidget *dw)
 {
+    qDebug() << Q_FUNC_INFO << dw->uniqueName();
     QScopedValueRollback<bool> guard(m_removeGuard, true);
     const int row = indexOf(dw);
     if (row == -1) {
@@ -224,6 +225,7 @@ void DockWidgetModel::remove(Controllers::DockWidget *dw)
 
         beginRemoveRows(QModelIndex(), row, row);
         m_dockWidgets.removeOne(dw);
+        qDebug() << "Removed one from " << row;
         endRemoveRows();
 
         Q_EMIT countChanged();