Hi,
I recently got hold of a Novation Launchpad to try, and was playing with custom feedback to light the leds.
I forgot to set the external input, and when clicking on Upper Value QLC just closed.
Easy to re-produce.
In VC create a button. Call up properties. Select Custom Feedback, and try to change either lower/upper values.
Linux Mint. QLC 4.10.2 git.