Unlock dialog doesn't show up when locking screen while monitor is detached (e.g. via KVM switch)

When there is no monitor attached while locking the screen with light-locker, the screen will be locked but the unlocking dialog does not show up. Instead, the system shows a black screen with the message "This session is locked. You'll be redirected to the unlock dialog automatically in a few seconds." (sometimes this message is only visible after going to another virtual console and back to [CTRL]+[ALT]+[F7]), but the unlock dialog never shows up. In that state, the session can only be unlocked using "sudo loginctl list-sessions" and then "sudo loginctl unlock-session ..." from a virtual console/SSH connection.

This issue can easily be reproduced by running a command like "sleep 30; light-locker-command --lock" and then detaching the VGA/DVI/HDMI monitor cable (and reconnecting it after the screen is locked).

The same also happens when using a KVM switch and switching over to another machine directly after triggering the screen lock. Since light-locker typically takes a couple of seconds from triggering the screen lock until the unlocking dialog shows up, it will frequently happen when switching to another system right after locking the screen.

Please note that this issue does not happen on a laptop since the integrated screen is always attached.

The problem has been verified with a default installation of XUbuntu 18.04 and 19.04.