I’ve updated the link above with this morning’s build. I think it’s a significant improvement.
If you’ve already downloaded and/or installed the beta, then RapidWeaver should inform you that beta 3 is available via auto-update.
And here’s a direct link, just in case.
https://yourhead.com/appcast/RW6/beta/Stacks4/Stacks_4.2.0b3_5457.zip
This latest build moves the stack rendering queue back to where it was just prior to v4.1.0 – and I think there’s a pretty good chance this is the source of the crash.