dolphin --daemon does not have the /dolphin/Dolphin_1 dbus path,
because it doesn't have any DolphinMainWindow.
Checking if the QDBusInterface is valid is not enough, we also need to look for dbus errors.
This should have been part of commit 7df5fc8edb54153f. Not sure how I missed it.
BUG: 408244
FIXED-IN: 19.08.0