Looks like this is a bug in 3.8.99, sorry! We've fixed it in v4 but unfortunately don't have plans to do another 3.8 release as we are focused on v4. Also we've hit 3.8.99 so we'd have to do a 3.8.100 release, a version number which doesn't look so nice. We'll post the v4 beta on the forum tomorrow.
BTW, we know not being able to double click to delete a key is very annoying. We didn't want to make a release with a version number like 3.8.100, so we re-released 3.8.99 with a fix. To get it you just need to delete your 3.8.99 file and let Spine download it again.