When i click on "pack islands" blender closes immediately. I didn't follow same steps exactly as Kent, i have already applied all mirror modifiers and most of the objects are not sharing mesh data. So i just selected everything , tab into edit mode,Average islands scale, Pack islands and blender shuts down completly.
here is a link to my blender file : https://gofile.io/d/OpYjpm
That's a bug!
Use a different version of Blender; I've tested your file in 2.83LTS, 2.90, 2.92 and it works there. Only in 2.91.0 Blender crashed when trying to Pack Islands.