I just tested version 1.2.1 on Aseprite 1.3.14.4 on macOS Sequoia 15.6 and didn't run into any issues. So the public beta may be the reason; however, I can't update to the public beta at this point because it's a work laptop.
I tried various combinations of settings for the extension. Could you maybe share yours?
Also, have you tried it with a different sprite? Try using a super basic 32x32 canvas with one diffuse, one normal, and one output layer.
Further, are you using any additional plugins? If so, could you try disabling them and then checking if it still causes a crash? Maybe by enabling them step by step, it could lead us to the root of the issue.
Thanks!