I'm in the process of trying sort it out as far as behavior, but there is some sort of bug with editing styles in 1.4.0svn.
When I have (intermittently) been able to produce it, was when I created a paragraph style, then created another one based on that one. Clicking on Done on one occasion, clicking on Apply on another caused Scribus to just disappear, no warning or anything. For now, I would suggest making sure you've saved your document before making/editing styles. Greg
