Rails 5.1: “unknown firstpos: NilClass” – Issue reloading application

I just faced exactly the same problem. I sovled it by setting:

config/environments/development.rb

from:

# Do not eager load code on boot.
config.eager_load = true

to:

**# Do not eager load code on boot.
config.eager_load = false

Hope this helps!
Cheers, Nic.