solve rails4 run rake 'undefined method 'existent' for nil:NilClass' exc...

Because Rails 4 `Rails.application.paths["db/seeds"]` retun nil. look https://github.com/rails/rails/blob/4-0-0/railties/lib/rails/engine/configuration.rb#L65 and https://github.com/rails/rails/blob/3-2-stable/railties/lib/rails/engine/configuration.rb#L59 .