Rails + Devise undefined method `errors' for nil:NilClass - ruby-on-rails

Question

I'm having issues loading a new registration page using Devise in my rails app. When I try to load the sign up page, I get the error:
undefined method `errors' for nil:NilClass
which seems to be caused by the following line:
<%= devise_error_messages! %>
What could I be doing wrong?
routes.rb:
devise_for :users, :controllers => {:registrations => :registrations, omniauth_callbacks: "users/omniauth_callbacks"}
devise_scope :user do
post 'registrations' => 'registrations#create', :as => 'register'
post 'sessions' => 'sessions#create', :as => 'login'
delete 'sessions' => 'sessions#destroy', :as => 'logout'
end
registrations_controller.rb
class RegistrationsController < Devise::RegistrationsController
# GET /user/sign_up
def new
build_resource({})
respond_with self.resource
end
The sign up link:
<%= link_to "Sign Up", new_registration_path(resource_name), :class=>"btn btn-mini", :style=>'float:left'%>
application_helper.rb (where resource_name is defined)
def resource_name
:user
end
def resource
#resource ||= User.new
end
def devise_mapping
#devise_mapping ||= Devise.mappings[:user]
end
the sign up view
<div class="signUpPage">
<div class="form_container">
<h3 class="form_heading sign_up">Sign up</h3>
<%= semantic_form_for(resource, :as => resource_name, :url => registration_path(resource_name)) do |f| %>
<%= devise_error_messages! %>
<div class="formField"><%= f.label :username, :class=> "label label-info" %><br />
<%= f.text_field :username, :class=> "signup_username", :maxlength=>"15"%></div>
<div class="usernameValidation validation">Username is already taken</div>
<div class="formField"><%= f.label :email, :class=> "label label-info" %><br />
<%= f.email_field :email%></div>
<div class="emailValidation validation">Email is already taken</div>
<div class="formField"><%= f.label :password, :class=>"label label-info" %><br />
<%= f.password_field :password, :class=> "signup_password" %></div>
<div class="passwordValidation validation">Password must be at least 8 characters</div>
<div class="formField"><%= f.label :password_confirmation, :class=>"label label-info" %><br />
<%= f.password_field :password_confirmation %></div>
<div class="passwordConfirmationValidation validation">Passwords do not match</div>
<div><%= f.submit "Sign up", :class=>"btn btn-info signup_button" %></div>
<% end %>
<%= render "devise/shared/links" %>
</div>
</div>
This is the full error message:
NoMethodError in Registrations#new
Showing Website/app/views/devise/registrations/new.html.erb where line #12 raised:
undefined method `errors' for nil:NilClass
Extracted source (around line #12):
9:
10: <%= semantic_form_for(resource, :as => resource_name, :url => registration_path(resource_name), namespace: 'sign_up', html: {id: 'form_user_sign_up'}) do |f| %>
11:
12: <%= devise_error_messages! %>
13:
14: <div class="formField"><%= f.label :username, :class=> "label label-info" %><br />
15: <%= f.text_field :username, :class=> "signup_username", :maxlength=>"15"%></div>
Rails.root: /Website
app/views/devise/registrations/new.html.erb:12:in `block in _app_views_devise_registrations_new_html_erb___65153715043208623_70149577578880'
app/views/devise/registrations/new.html.erb:10:in `_app_views_devise_registrations_new_html_erb___65153715043208623_70149577578880'
app/controllers/registrations_controller.rb:11:in `new'