If you are talking about today’s update Glyphs 3.5 (3500), I already reported about an issue when plugins can’t identify the active tool.
Also, when something like that happens and your work has stopped, you can re-download the previous application version (or any other version you like) from the official website, just like I did this morning. Or, you can even keep a few versions of the application, like stable version and cutting edge version, so you always have a fallback if something goes wrong.