There's a bug in Ubuntu 18.04 and 18.04 which causes the Ubuntu dock/launcher to be shown on the lock screen in some cases. What's more, it's possible to close running applications or remove icons from the Ubuntu dock, even though the screen is locked. This bug was fixed in Ubuntu 19.04 Disco Dingo. The fix will probably be backported to Ubuntu 18.04 and 18.10, but if you don't want to wait, you can apply the patch right now on your Ubuntu 18.04 or 18.10 desktop.
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