Not quite sure if what you are reporting is a bug, or standard warning from self-signed issuance. If you are to assign the FQDN an A Record at your DNS provider, it would still produce an error in regards to the cert not being issued by a valid authority.
Although, this does annoy myself and I know there are others out there that are put off by the message, though it is common place with self-signing. This has brought me to potentially providing LetsEncrypt as a function for setting up a proper and free SSL during script install. I might even separate parts of this script for users whom already have QuickBox installed. The only thing that would be needed at that point is a fully qualified domain, as well as a Free DNS provider (of which I recommend CloudFlare)
I think there may even be free domain providers in the form of proxy/sub-domain services (assuming the name isn't taken)
There could additionally, be a function in the script that asks for an FQDN as well as building the actual SSL, rather than this being used from hosts and the ssl silently self-signed on install.