I tried above but does not crashes.
When I put vncserver autostart option vnc server is not started. And then I start GUI with startx and one error appears:
“GDBus.error.org.freedesktop.PolicyKit1.Error.Failed: An authentication agent already exists for the given subject”
Does it have any relation with vncserver not starting?
New reference code from bugreport: 8ac2451e8073-1