wlr/taskbar won't exec on-update when all windows are closed

[AlvinLIQ]

I have a custom global title bar looks like:
"custom/title": {
"exec": "echo get the active window title",
"signal": 18
}
and I set wlr/taskbar on-update to "pkill -RTMIN+18 waybar" in order to update the title

but when I close all windows it still shows previous active window title.
I wonder is there a better way to do this?