[DiapDealer]

Part of the reason trying this occurred to me is that of the three dockwidgets that mainwindow loads/shows by default (Preview, BookBrowser and ToC), ToC was the only one that didn't have an explicit destructor function. I realize there doesn't necessarily need to be one (the Clips and Validator dockwidgets come to mind), but I was thinking maybe there was something that wasn't getting properly cleaned up (because of a potential defect in that version of Qt) that perhaps explicitly deleting would take care of.

I might take some time to reorder those explicit deletes for mac and see if there is indeed an order that can produce adverse effects in Linux with Qt 5.9.5.

At this point, I think I'm writing off the segfault as something that's not getting cleaned up in 5.9.5 that doesn't really matter since Sigil is being destroyed anyway.