Addendum: write a useful exception message for future travelers that might need to review that stack. What was null? Why might it be null? Mis-configuration?

Addendum: write a useful exception message for future travelers that might need to review that stack. What was null? Why might it be null? Mis-configuration? Missing configuration? Can't load some data or connect to some system? Which setting should be verified? Give simple hints on what might have gone wrong to help future you.