Ruby on Rails

Sns::Application.routes.draw do
  root :to => "questions#index"
  resources :questions do
    resources :question_responses
end

class Question < ActiveRecord::Base
  validates :title, :presence => true
  validates :description, :presence => true
  
  has_many :question_responses, :dependent => :destroy, :class_name => "QuestionResponse"
  accepts_nested_attributes_for :question_responses, :allow_destroy => true
end

class QuestionResponse < ActiveRecord::Base
  validates :body, :presence => true
  belongs_to :question, :foreign_key => "question_id"
end

class QuestionsController < ApplicationController
  before_filter :find_question
  before_filter :find_question_response, :only => [:show, :edit, :update, :destroy]
  
  def index
    @question = Question.new
    @questions = Question.all
  end
  
  def new
    @question = Question.new
    @question_response = @question.question_responses.build
  end
  
  def create
    @question = Question.new(params[:question])
    
    if @question.save
      flash[:notice] = "Question has been created."
      redirect_to @question
    else
      flash[:alert] = "Question has not been created."
      render :action => 'new'
    end
    
    @question_response = @question.question_responses.build(params[:question_response])
    if @question_response.save
      flash[:notice] = "Reply has been created."
      redirect_to [@question, @question_response]
    else
      flash[:alert] = "Reply has not been created."
      render "question_responses/new"
    end
  end
  
  def show
    @question = Question.find(params[:id])
  end
  
  def edit
    @question = Question.find(params[:id])
  end
  
  def update
    @question = Question.find(params[:id])
    
    if @question.update_attributes(params[:question])
      flash[:notice] = "Question has been updated."
      redirect_to @question
    else
      flash[:alert] = "Question has not been updated."
      render :action => 'edit'
    end
  end
  
  def destroy
    @question = Question.find(params[:id])
    @question.destroy
    flash[:notice] = "Question has been deleted."
    redirect_to questions_path
  end
  
  def find_question_response
    @question_response = @question.question_responses.find(params[:id])
  end
  
  private
  def find_question
    if (params[:question_response] && params[:question_response][:question_id])
      @question = Question.find(params[:question_response][:question_id])
    elsif params[:question_id]
      @question = Question.find(params[:question_id])
    end
  end
end

undefined method `question_responses' for nil:NilClass

app/controllers/questions_controller.rb:55:in `find_question_response'
activesupport (3.2.1) lib/active_support/callbacks.rb:429:in `_run__35842215__process_action__9839693__callbacks'
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/home/administrator/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/webrick/httpserver.rb:138:in `service'
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/home/administrator/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/webrick/server.rb:191:in `block in start_thread'