Sorry for not getting back...my testing led to further testing...
So, I tried two other computers in my house - both Windows 10 - and neither of them showed this problem

Neither of them had Sigil installed before, it was a fresh install of 1.0.0 and it worked as advertised.
I went into my computer and removed QT, and python 3.7 - restarted the computer and Sigil crashes.
The QTBUG-77385 you referenced does sound similar-ish. Hopefully it works - I don't relish wiping the hard drive and doing a fresh install!
Thanks for checking into it again.