Comment 40 for bug 1769383

Looking at the gnome-shell JS code referenced by the stack trace, it appears it is indeed disabling extensions when locking the screen.

The screenShield.js code handles the lock screen, and pushes and pops a "session mode" of "lock-screen". In sessioMode.js, the "lock-screen" mode is defined with allowExtensions=false. Finally, extensionSystem.js watches session mode changes and calls the various extension enable/disable callbacks as needed.

So somehow ubuntu-dock has a stale window in Transparency._trackedWindows that triggers an exception while trying to disable the dock when locking the screen. It fails to fully disable the dock, so it displays over the top of the lock screen.