In Ubuntu unity tray icon geometry returns QRect(0,0 0x0), works fine in windows

I need to get the tray icon geometry so I can put a dialog window near it. Using Qt 4.8.0 in windows 7 the problem doesn't occur and the geometry is as expected but in a new Ubuntu 12.04 the geometry returns QRect(0,0 0x0) and the dialog windows appears in the corner of the screen.

The code to move the dialog window:

void IDialog::moveToSystrayArea(const QSystemTrayIcon * trayIcon)
{
qDebug() << trayIcon->geometry();
if (trayIcon)
move(trayIcon->geometry().left() - geometry().width()/2,
trayIcon->geometry().top() - geometry().height());
}

Obs: works fine in windows 7 (32,64 bits)