Adding QToolBar makes QMainWindow disappear

On Mac OSX.

#include <QtGui/QApplication>
#include <QtGui/QMainWindow>
#include <QtGui/QToolBar>

int main(int argc, char** argv)
{
QApplication app(argc, argv);
QMainWindow mw;
mw.show();
// mw is shown now

// makes the mw disappear
QToolBar* tb = new QToolBar("title", &mw);

app.exec();
}