lightdm crashes when unlocking system- User attempting to unlock is logged out

[The Computer Geek]

Sometimes when I go to unlock my system, lightdm will crash and log me out. This is infuriatingly annoying. This usually happens when my system is either locked and then left alone for over 20 minutes, or when my system locks due to inactivity.

Ubuntu version: 16.04.3 LTS
lightdm version: 1.18.3-0ubuntu1.1 500
What I expect to happen:
1) I lock the computer or it locks on it's own
2) It's left alone for over 20 minutes
3) I go to unlock
4) It unlocks and everything's open

What actually happens:
1) I lock the computer or it locks on it's own
2) It's left alone for over 20 minutes
3) I go to unlock
4) The screen goes black for 2 to 5 seconds, then comes back up a login prompt
5) I login
6) I get a desktop, and everything's been closed

This is the output of /var/log/lightdm/lightdm.log after the incident:

[+93016.79s] DEBUG: Continue authentication
[+93016.80s] DEBUG: Session pid=13591: Authentication complete with return value 0: Success
[+93016.80s] DEBUG: Authenticate result for user the-computer-geek: Success
[+93016.80s] DEBUG: User the-computer-geek authorized
[+93016.81s] DEBUG: Greeter sets language en_US
[+93016.86s] DEBUG: Greeter requests session xubuntu
[+93016.86s] DEBUG: Seat seat0: Returning to existing user session the-computer-geek
[+93016.86s] DEBUG: Unlocking login1 session c12
[+93016.86s] DEBUG: Activating VT 7
[+93016.96s] DEBUG: Seat seat0: Stopping greeter
[+93016.96s] DEBUG: Session pid=13506: Sending SIGTERM
[+93016.96s] DEBUG: Activating login1 session c12
[+93016.97s] DEBUG: Session pid=13591: Exited with return value 0
[+93016.97s] DEBUG: Seat seat0: Session stopped
[+93016.97s] DEBUG: Seat seat0 changes active session to c12
[+93016.97s] DEBUG: Session c12 is already active
[+93016.97s] DEBUG: Session pid=13506: Exited with return value 0
[+93016.97s] DEBUG: Seat seat0: Session stopped
[+93016.97s] DEBUG: Seat seat0: Stopping display server, no sessions require it
[+93016.97s] DEBUG: Sending signal 15 to process 13492
[+93017.03s] DEBUG: Process 13492 exited with return value 0
[+93017.03s] DEBUG: DisplayServer x-1: X server stopped
[+93017.03s] DEBUG: Releasing VT 8
[+93017.03s] DEBUG: DisplayServer x-1: Removing X server authority /var/run/lightdm/root/:1
[+93017.03s] DEBUG: Seat seat0: Display server stopped
[+93021.37s] DEBUG: Process 7864 terminated with signal 6
[+93021.37s] DEBUG: DisplayServer x-0: X server stopped
[+93021.37s] DEBUG: Releasing VT 7
[+93021.37s] DEBUG: DisplayServer x-0: Removing X server authority /var/run/lightdm/root/:0
[+93021.37s] DEBUG: Seat seat0: Display server stopped
[+93021.37s] DEBUG: Seat seat0: Stopping session
[+93021.37s] DEBUG: Session pid=8073: Sending SIGTERM
[+93021.37s] DEBUG: Seat seat0: Active display server stopped, starting greeter
[+93021.37s] DEBUG: Seat seat0: Creating greeter session
[+93021.37s] DEBUG: Seat seat0: Creating display server of type x
[+93021.37s] DEBUG: Using VT 7
[+93021.37s] DEBUG: Seat seat0: Starting local X display on VT 7
[+93021.37s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log
[+93021.37s] DEBUG: DisplayServer x-0: Writing X server authority to /var/run/lightdm/root/:0
[+93021.37s] DEBUG: DisplayServer x-0: Launching X Server
[+93021.37s] DEBUG: Launching process 17470: /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+93021.37s] DEBUG: DisplayServer x-0: Waiting for ready signal from X server :0
[+93021.96s] DEBUG: Session pid=8073: Exited with return value 0
[+93021.96s] DEBUG: Seat seat0: Session stopped
[+93021.96s] DEBUG: Seat seat0: Stopping display server, no sessions require it
[+93021.97s] DEBUG: Got signal 10 from process 17470
[+93021.97s] DEBUG: DisplayServer x-0: Got signal from X server :0
[+93021.97s] DEBUG: DisplayServer x-0: Connecting to XServer :0
[+93021.97s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+93021.98s] DEBUG: Session pid=17605: Started with service 'lightdm-greeter', username 'lightdm'
[+93021.98s] DEBUG: Session pid=17605: Authentication complete with return value 0: Success
[+93021.98s] DEBUG: Seat seat0: Session authenticated, running command
[+93021.98s] DEBUG: Session pid=17605: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/lightdm-gtk-greeter
[+93021.98s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+93021.98s] DEBUG: Session pid=17605: Logging to /var/log/lightdm/seat0-greeter.log
[+93022.00s] DEBUG: Activating VT 7
[+93022.00s] DEBUG: Activating login1 session c16
[+93022.00s] DEBUG: Seat seat0 changes active session to c16
[+93022.00s] DEBUG: Session c16 is already active
[+93022.07s] DEBUG: Greeter connected version=1.18.3 resettable=false
[+93022.22s] DEBUG: Greeter start authentication for the-computer-geek
[+93022.22s] DEBUG: Session pid=17678: Started with service 'lightdm', username 'the-computer-geek'
[+93022.22s] DEBUG: Session pid=17678: Got 1 message(s) from PAM
[+93022.22s] DEBUG: Prompt greeter with 1 message(s)
[+93027.12s] DEBUG: Continue authentication
[+93027.13s] DEBUG: Session pid=17678: Authentication complete with return value 0: Success
[+93027.13s] DEBUG: Authenticate result for user the-computer-geek: Success
[+93027.13s] DEBUG: User the-computer-geek authorized
[+93027.13s] DEBUG: Greeter sets language en_US
[+93027.17s] DEBUG: Greeter requests session xubuntu
[+93027.17s] DEBUG: Seat seat0: Stopping greeter; display server will be re-used for user session
[+93027.17s] DEBUG: Session pid=17605: Sending SIGTERM
[+93027.18s] DEBUG: Greeter closed communication channel
[+93027.18s] DEBUG: Session pid=17605: Exited with return value 0
[+93027.18s] DEBUG: Seat seat0: Session stopped
[+93027.18s] DEBUG: Seat seat0: Greeter stopped, running session
[+93027.18s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session4
[+93027.18s] DEBUG: Session pid=17678: Running command /usr/sbin/lightdm-session startxfce4
[+93027.18s] DEBUG: Creating shared data directory /var/lib/lightdm-data/the-computer-geek
[+93027.18s] DEBUG: Session pid=17678: Logging to .xsession-errors
[+93027.22s] DEBUG: Activating VT 7
[+93027.22s] DEBUG: Activating login1 session c17
[+93027.22s] DEBUG: Seat seat0 changes active session to c17
[+93027.22s] DEBUG: Session c17 is already active
[+93381.89s] DEBUG: Seat seat0: Locking
[+93381.89s] DEBUG: Seat seat0: Creating greeter session
[+93381.89s] DEBUG: Seat seat0: Creating display server of type x
[+93381.89s] DEBUG: Using VT 8
[+93381.89s] DEBUG: Seat seat0: Starting local X display on VT 8
[+93381.89s] DEBUG: DisplayServer x-1: Logging to /var/log/lightdm/x-1.log
[+93381.89s] DEBUG: DisplayServer x-1: Writing X server authority to /var/run/lightdm/root/:1
[+93381.90s] DEBUG: DisplayServer x-1: Launching X Server
[+93381.90s] DEBUG: Launching process 19411: /usr/bin/X -core :1 -seat seat0 -auth /var/run/lightdm/root/:1 -nolisten tcp vt8 -novtswitch
[+93381.90s] DEBUG: DisplayServer x-1: Waiting for ready signal from X server :1
[+93382.01s] DEBUG: Seat seat0 changes active session to
[+93382.50s] DEBUG: Process 19411 terminated with signal 6
[+93382.50s] DEBUG: DisplayServer x-1: X server stopped
[+93382.50s] DEBUG: Releasing VT 8
[+93382.50s] DEBUG: DisplayServer x-1: Removing X server authority /var/run/lightdm/root/:1
[+93382.50s] DEBUG: Seat seat0: Display server stopped
[+93382.50s] DEBUG: Seat seat0: Stopping session
[+93382.50s] DEBUG: Seat seat0: Session stopped
[+93382.50s] DEBUG: Seat seat0: Stopping display server, no sessions require it
[+93382.50s] DEBUG: Seat seat0: Active display server stopped, starting greeter
[+93382.50s] DEBUG: Seat seat0: Creating greeter session
[+93382.50s] DEBUG: Seat seat0: Creating display server of type x
[+93382.50s] DEBUG: Using VT 8
[+93382.50s] DEBUG: Seat seat0: Starting local X display on VT 8
[+93382.50s] DEBUG: DisplayServer x-1: Logging to /var/log/lightdm/x-1.log
[+93382.50s] DEBUG: DisplayServer x-1: Writing X server authority to /var/run/lightdm/root/:1
[+93382.50s] DEBUG: DisplayServer x-1: Launching X Server
[+93382.50s] DEBUG: Launching process 19419: /usr/bin/X -core :1 -seat seat0 -auth /var/run/lightdm/root/:1 -nolisten tcp vt8 -novtswitch
[+93382.50s] DEBUG: DisplayServer x-1: Waiting for ready signal from X server :1
[+93382.86s] DEBUG: Got signal 10 from process 19419
[+93382.86s] DEBUG: DisplayServer x-1: Got signal from X server :1
[+93382.86s] DEBUG: DisplayServer x-1: Connecting to XServer :1
[+93382.89s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+93382.89s] DEBUG: Session pid=19434: Started with service 'lightdm-greeter', username 'lightdm'
[+93382.90s] DEBUG: Session pid=19434: Authentication complete with return value 0: Success
[+93382.90s] DEBUG: Seat seat0: Session authenticated, running command
[+93382.90s] DEBUG: Session pid=19434: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/lightdm-gtk-greeter
[+93382.90s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+93382.90s] DEBUG: Session pid=19434: Logging to /var/log/lightdm/seat0-greeter.log
[+93382.95s] DEBUG: Activating VT 8
[+93382.95s] DEBUG: Locking login1 session c17
[+93382.95s] DEBUG: Activating login1 session c18
[+93382.95s] DEBUG: Seat seat0 changes active session to c18
[+93382.95s] DEBUG: Session c18 is already active
[+93383.03s] DEBUG: Greeter connected version=1.18.3 resettable=false
[+93383.06s] DEBUG: User /org/freedesktop/Accounts/User108 added
[+93383.18s] DEBUG: Greeter start authentication for the-computer-geek
[+93383.18s] DEBUG: Seat seat0: Failed to work out session ID to mark
[+93383.18s] DEBUG: Session pid=19518: Started with service 'lightdm', username 'the-computer-geek'
[+93383.18s] DEBUG: Session pid=19518: Got 1 message(s) from PAM
[+93383.18s] DEBUG: Prompt greeter with 1 message(s)

I don't know whether to file this as a security issue, but the only workaround to this issue is to either log off when the system is left alone for >20 minutes or disable automatic locking and never lock the system.