Just don't do that!
I kid (sort of). The Reports tool is intended to be run against a "current" snapshot of the epub. Changing the underlying epub will, of course, require that the Reports tool be rerun to account for what's changed.
It's definitely not a Qt issue. About the only thing we could do to prevent the crash would be to make the Reports dialog modal. Meaning you wouldn't be able to interact with the rest of Sigil until you dismissed the Reports dialog.
We'll take it under advisement. In the meantime ... just don't do that!