Something in your Sigil.ini file must be hosed.
Double click the titlebar of the Preview widget to re-dock it somewhere, and then make sure Sigil can open/close without freezing with it displayed (but docked) before trying to undock it again.
Myself; I'd just delete the Sigil.ini file, let Sigil recreate it and then set everything back up how I like it again. But I know people tend to fall in love with their heavily customized settings and resist doing that for as long as they possibly can. Heck, I delete my Sigil.ini file at least once a week these days (with no backup). Sometimes just for the practice. Ain't no big deal.
