Crashing bug are always the worst because they make the user feel like they can't trust the program. And crashing bugs that prevent you from even opening your data file are the worst of the worst. A helpful user just reported one to me, and sent a test case, which is always good.
If you put nothing but white space in a field, and then save the file. Bookcase will crash whenever you open the file again. That seems like a weird bug, and it is. For some reason, Qt returns a DomNode that is not a textNode when the node has nothing but spaces. Don't ask me why, probably some weird reasoning in the XML spec.
So I fixed the bug, and the next version won't have this problem. Until then, don't put empty space in your Paragraph fields!