This happens in BS 5.8.0 (linux appimage) and Kubuntu 20.04
When I hover over a yellow notice next to an overridden style at the “Styles” panel and click the “See Block” button I get the following message window
Error KIO Client
Unable to run the command specified. The file or folder /tmp/.mount_BootstlFcAn6/resources/app.asar/app/index.html does not exist.
It seems to work functionally but I see that error message every time.