qtvcp: close dbus bus when sys_notify init fails

If `init()` constructs `dbus.SessionBus(mainloop)` with the
PyQt5/PyQt6 mainloop integration and the subsequent
`bus.get_object(org.freedesktop.Notifications, ...)` call raises
(e.g. `org.freedesktop.DBus.Error.ServiceUnknown` when no
notification daemon owns the name), the Python reference to `bus`
is dropped on the exception, but dbus-python's mainloop layer has
already attached a `QSocketNotifier` to the connection fd. That
notifier keeps the C-level connection alive in a half-initialized
state. On the next Qt event-loop tick, the queued reply is
dispatched via `dbus_connection_dispatch()` and segfaults inside
`_dbus_list_unlink` because the connection's internal queue is no
longer in a consistent state.

Close the bus explicitly on the failure path so the socket
notifier detaches and the connection is released cleanly. The
`Desktop Notify not available` warning continues to be the only
user-visible side effect.

Reproduced on Ubuntu 24.04 CI under xvfb + offscreen Qt where no
session-bus notification daemon is present; gdb -batch captured a
Thread 1 backtrace ending in `dbus_connection_dispatch` ->
`_dbus_list_pop_first_link` -> `_dbus_list_unlink` (segv).

Author: grandixximo

lib/python/qtvcp/lib/sys_notify.py

@@ -62,6 +62,7 @@ def init(app_name):
     interface = "org.freedesktop.Notifications"
 
     mainloop = None
+    bus = None
     try:
         if DBusQtMainLoop is not None:
             mainloop = DBusQtMainLoop(set_as_default=True)
@@ -76,6 +77,23 @@ def init(app_name):
             DBUS_IFACE.connect_to_signal('NotificationClosed', _onNotificationClosed)
     except Exception as e:
         LOG.warning('Desktop Notify not available:: {}'.format(e))
+        # When the SessionBus is constructed with a PyQt5/PyQt6 mainloop
+        # integration, dbus-python installs a QSocketNotifier on the
+        # connection fd. If the subsequent get_object/Interface setup
+        # raises (e.g. ServiceUnknown when no notification daemon owns
+        # org.freedesktop.Notifications), the Python-side bus reference
+        # is dropped on exception but the QSocketNotifier keeps the
+        # underlying C connection alive in a half-initialized state.
+        # The next QEventLoop tick dispatches a queued message via
+        # dbus_connection_dispatch() and segfaults inside
+        # _dbus_list_unlink. Close the bus explicitly so the notifier
+        # detaches and the connection is fully released.
+        DBUS_IFACE = None
+        if bus is not None:
+            try:
+                bus.close()
+            except Exception:
+                pass
 
 
 def _onActionInvoked(nid, action):