I have written this generator code but it returns 'can't convert nil into String' when I call m.directory inside the manifest. Anyone know what had happened?

```
class AuthGenerator < Rails::Generator::NamedBase
  attr_reader :user_class_name
  def initialize(runtime_args, runtime_options={})
    @user_class_name="User"
    @controller_class_name="AccountController"
    @user_class_file_name="#{@user_class_name}.rb"
    @controller_class_file_name="#{@controller_class_name}.rb"
  end

  def manifest
    record do |m|
      m.class_collisions @controller_class_name, @user_class

      puts @user_class_name
      m.directory File.join('app/models', @user_class_name)

    end
  end

```

end