Thanks for the info Kovid.
At the time I wrote this plugin it was more about getting it working as quickly as possible - which meant copying how calibre approached presenting the user with a font choice. I didn't want to go through my own pain curve of hitting issues that you presumably had solved over the years with calibre.
It is also a tradeoff. I am not a huge fan of popups off popups, which is what you get by having a font dialog window launched from a plugin window. And in this plugin where you have the preview of the cover image displayed it is more convenient (I think) to quickly stab the spin up/down boxes to make some text fit to your liking than repeatedly open and close font dialogs.
So given it has never crashed for me and I have a teeny fraction of the userbase that you have to support for the slim chance of it crashing for someone else using this plugin I can live with it as is for now and have a weekend off development instead
