Avoid crashing when DBus isn't running
If DBus isn't running QDBusConnection::sessionBus().interface() and QDBusConnection::systemBus().interface() return nullptr, so dereferencing it will crash immediately.
I am currently trying to run KWin on a new architecture where DBus hasn't been ported yet. This allows me to start KWin without crashing immediately. I am aware that there will probably be some missing functionality, but all basic functionality I've tested does appear to work.