It's very easy to reproduce:

QMenu trayIconMenu;
trayIconMenu.addAction("do something");
QSystemTrayIcon* trayIcon = new QSystemTrayIcon();
trayIcon->show();
trayIcon->setContextMenu(&trayIconMenu); // Fails here

In application output we can see:

QSystemTrayIcon::setVisible: No Icon set
service is "org.kde.StatusNotifierItem-12650-1"
Registering a client interface to the KStatusNotifierWatcher
The program has unexpectedly finished.

In Plasma Desktop 4 works just fine, in XFCE too. Without setting context menu everything works fine as well, even in Plasma Desktop 5!

I've attached tiny test project and screenshots.