I am having the same issue as well! I think it has something to do with HealthKit queries on my end, but very annoying issue as it sometimes works for a few minutes without happening, and other times it's instant on launch,
Error code:
2020-10-21 15:54:53.055674+0200 Heart Rate WatchKit Extension[448:36709] Fatal error: Can't remove first element from an empty collection: file Swift/RangeReplaceableCollection.swift, line 624