Google threw me a curve ball here. In 3.6.x I switched to the Google-blessed Material Components date/time picker dialogs in the hope of getting better support and them being better integrated (and nicer looking). However, turns out you have to manually test every android release and make sure that its not crashing (even if you use them exactly as they intended).
So I apologize if you were impacted by this problem (and opening the due or defer date/time picker caused the app to crash for you). This should not be evened out.
- FIX: fix crash on Android 9 devices when opening the date/time picker
As always, the version has been released to the testers group and will be rolled out to the public in the coming days (unless problems occur).
Check out the introductory post regarding the new date/time pickers FocusGTD 3.6.1 release announcement