Are you crashing when trying to double-click a previously openable script document on the Mac?
This can happen if you are trying to open a Screenwriter .mmx (versus a .mmsw) document after you've upgraded from Screenwriter 18.104.22.168 to Screenwriter 6.5, or if you downgrade from Screenwriter 6.5 or 6.3 to Screenwriter 22.214.171.124.
Most likely you are accidentally running the WRONG version of Screenwriter. When you double-click on your script in the .mmx (Screenwriter XML format) document you are launching Screenwriter 126.96.36.199, when you want to be running Screenwriter 6.5.
Look in your Applications folder. You should see TWO Screenwriter folders: one for Screenwriter 6 and one for Screenwriter 6.5.
DELETE the ENTIRE /Applications/Screenwriter 6 folder. Trash it, and EMPTY the trash. This will NOT delete any of your script documents -- they should be located elsewhere.
Now, when you double-click on your .mmx document it should launch without crashing.