code> QMainWindow 。如果是，则需要设置一个centralWidget：

I suppose your class specializes QMainWindow. If so, it needs a centralWidget to be set:

MainWindow::MainWindow()
{
    // added by jpo38
    QWidget* mainWidget = new QWidget( this );
    setCentralWidget( mainWidget );
    // end added by jpo38

    mainLayout = new QHBoxLayout(mainWidget);
    tabber = new QTabWidget(mainWidget);
    analysisTreeView = new QTreeView(mainWidget);

    tabber->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

    mainLayout->addWidget(tabber, 0);
    mainLayout->addWidget(analysisTreeView, 0);

    createActions();
    createMenus();
    createToolBars();

    connect(tabber, SIGNAL(currentChanged(int)), this, SLOT(currentTabChanged(int)));

    setLayout(mainLayout);
}