NoMethodError: undefined method `stringify_keys' for nil:NilClass

[jgraichen]

Rails 6.1 without config file.

NoMethodError: undefined method `stringify_keys' for nil:NilClass

      mnemosyne-ruby-1.13.0/lib/mnemosyne/railtie.rb:8:in `block in <class:Railtie>'
      railties-6.1.7.4/lib/rails/initializable.rb:32:in `instance_exec'
      railties-6.1.7.4/lib/rails/initializable.rb:32:in `run'
      railties-6.1.7.4/lib/rails/initializable.rb:61:in `block in run_initializers'