There is a long-standing crash bug in Mac Screenwriter that goes all the way back to the early versions of Screenwriter 6:
If you type any character, at all, while Screenwriter is launching or opening a .mmsw document, Screenwriter will crash. (this doesn't seem to affect Screenwriter 6.5 .mmx documents)
It's that simple.
This affects double-clicking on the Screenwriter app, double-clicking on a document, opening a document using the FILE > OPEN menu command, or selecting a recently-opened document from the bottom of the FILE menu. Start typing before the document has been fully opened, and BANG!
We've corrected this in a yet-to-be-posted version of Screenwriter 6.5, version 6.5.4.
Of course, the solution (for now) is equally simple: don't type until you see the document has completely opened.