Thank you!
Yes, according to your debug log, what I added will not help!
The std::runtime_error when Sigil tries to read the missing NCX is being caught in the right place and the Error Dialog is shown. So once you hit the ok button on the Error Dialog, it should have returned and run 34, but instead Sigil is closing or crashing ... but I can not see why ....
Especially as this only happens when Sigil is not already running which makes this even stranger.
I noticed from your debug output you are running Sigil as Maximized. That changes the startup sequence somewhat. Could you try one more time with Sigil's MainWindow NOT Maximized to see if that impacts anything?