Accessibility: Do not use the session bus if not connected

When there is no DBus session, there will be no Linux accessibility,
since it relies on the presence of DBus.

Fixes: QTBUG-50189
Fixes: QTBUG-51940
Change-Id: I7503011b39ba2a806ddc12e89d0f7bd72a628b64
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

src/platformsupport/linuxaccessibility/dbusconnection.cpp

@@ -71,6 +71,10 @@ DBusConnection::DBusConnection(QObject *parent)
 {
     // Start monitoring if "org.a11y.Bus" is registered as DBus service.
     QDBusConnection c = QDBusConnection::sessionBus();
+    if (!c.isConnected()) {
+        return;
+    }
+
     dbusWatcher = new QDBusServiceWatcher(A11Y_SERVICE, c, QDBusServiceWatcher::WatchForRegistration, this);
     connect(dbusWatcher, SIGNAL(serviceRegistered(QString)), this, SLOT(serviceRegistered()));