after uninstall gnome-shell-extensions and update gnome-shell-extension-ubuntu-dock package, the lock screen is normal now without visible dock on locked screen.
....
gnome-shell-extension-ubuntu-dock/bionic-updates,bionic-updates,now 0.9.1ubuntu18.04.3 all [installed,automatic]
....
after uninstall gnome-shell- extensions and update gnome-shell- extension- ubuntu- dock package, the lock screen is normal now without visible dock on locked screen. extension- ubuntu- dock/bionic- updates, bionic- updates, now 0.9.1ubuntu18.04.3 all [installed, automatic]
....
gnome-shell-
....