Is kill $(lsof -t -i:8000) really the propper way to stop php -S localhost:8000 kirby/router.php ?
Seems quiet harsh 
You can use CTRL+C
In my defense: 
That’s what I tried first but had a conflict with another app. So I deleted the shortcut at the other app. And that didn’t help. That is why I came here. But meanwhile, after restarting tho whole computer, issue was gone. Again the old turn-on-off-trick.