Reproduced consistently in multiple blank projects. Type a word, enter Composition/Fullscreen mode, then right click on the word and select ‘Dictionary’. Upon closing the pop-up window, Scrivener immediately closes without producing an error message or saving changes to the project.
Reproduced on Windows 10 Home Edition, Scrivener Version 2.9.0.1 Beta (19429) - 20 Nov 2017