Comment 16 for bug 1291526

nvidia-prime doesn't use dbus, not directly at least (it can use policykit on demand)

What I see from the backtrace is that ksmserver gets an _XIOError, which I assume ksmserver handles with its own Xio_ErrorHandler(). Returning from the error handler causes ksmserver to exit. If you want to get around this, you can either use xcb (instead of xlib), or do what they suggest here:
http://lists.x.org/archives/xorg-devel/2010-July/011539.html