I’ve heard several reports of the app crashing after enabling the Reminders integration (see How to Import your to do list into Structured) and then always crashing on subsequent opens.
Currently, the only workaround (I know of) is disabling Reminders for Structured. To do so, go into the Settings app, search for Structured and disable the Reminders switch.
The issue seems to be caused by some reminders, but I can’t yet reproduce it. If you have any info on this, please let me know at email@example.com.