Did this this latest test build run before and does it crash now? Do you recall if you had "Ignore Illegal Memory Accesses" enabled in preferences (as it should be)?
Show us the content of your prefs file:
Launch Terminal (in /Application/Utilities/) and type at the prompt
followed by a return.
The file will open in your default text editor. Select all content and copy and paste it here in your reply.
Edit 1:
If you have backups (Time Machine or otherwise) you can easily recover a previous version of the corrupted DMG.
Edit 2:
PS. Do you have a build that still has Dynamic screen refresh rates?
All builds since many years can be and should be set to Dynamic screen refresh, also the latest test builds. A difference that you may notice is that SheepShaver now uses its own ("software") cursor inside its window and not the host's ("hardware") cursor.