Rails Ancestry undefined method `model_name' for NilClass:Class - ruby-on-rails

Question

I'm trying to learn how to use the "Ancestry" gem. I watched the related episode in Railscast and get the code. The message table works fine. But I'm trying to duplicate it with a new table called locations. I've copied and changed names on all the code (I believe).
I created the first location. But, when I go to localhost:3000/locations, I get undefined methodmodel_name' for NilClass:Classfor_form.html.erb` line:
<%= form_for #location do |f| %>
routes.rb:
Messenger::Application.routes.draw do
resources :locations
resources :messages
root :to => 'messages#index'
end
_form.html.erb:
<%= form_for #location do |f| %>
<%= f.error_messages %>
<%= f.hidden_field :parent_id %>
<p>
<%= f.label :locname, "New Location" %><br />
<%= f.text_area :locname, :rows => 8 %>
</p>
<p><%= f.submit "Post Location" %></p>
<% end %>
index.html.erb:
<% title "Locations" %>
<%= nested_locations #locations.arrange(:order => :created_at) %>
<%= render "form" %>
locations_controller.rb:
class LocationsController < ApplicationController
def index
#locations = Location.scoped
#location = Location.new
end
def show
#location = Location.find(params[:id])
end
def new
#location = Location.new(:parent_id => params[:parent_id])
end
def create
#location = Location.new(params[:location])
if #location.save
redirect_to locations_url
else
render :new
end
end
def destroy
#location = Location.find(params[:id])
#location.destroy
redirect_to locations_url
end
end
locations_helper.rb:
module LocationsHelper
def nested_locations(locations)
locations.map do |location, sub_locations|
render(location) + content_tag(:div, nested_locations(sub_locations), :class => "nested_locations")
end.join.html_safe
end
end