NoMethodError Rails 3.2

I've been searching all over and can't find the answer to this anywhere.

I'm taking the intro to RoR course on udemy, and I've been able to solve all the problems I've had in the first 80% of the course, but now I'm at a roadblock and can't find this. We're building an app like Etsy, and I'm at the point where I need to restrict users from editing/deleting listings that don't belong to them.

I'm running Ruby 1.9.3 on Rails 3.2.21

I tried following the instructions for adding the check user filter, but when I checked back on local host, I received this error:

NoMethodError in ListingsController#edit

undefined method `user' for nil:NilClass

app/controllers/listings_controller.rb:98:in `check_user'

Parameters: {"id"=>"8"}

My code matches the instructor's code exactly, but I think this error is because I'm using Rails 3, and he's using 4.

Here's my listings_controller.rb

class ListingsController < ApplicationController
  # GET /listings
  # GET /listings.json
  before_filter :authenticate_user!, only: [:new, :create, :edit, :update, :destroy]
  before_filter :check_user, only: [:edit, :update, :destroy]

  def index
    @listings = Listing.all

    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @listings }
    end
  end

  # GET /listings/1
  # GET /listings/1.json
  def show
    @listing = Listing.find(params[:id])

    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @listing }
    end
  end

  # GET /listings/new
  # GET /listings/new.json
  def new
    @listing = Listing.new

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @listing }
    end
  end

  # GET /listings/1/edit
  def edit
    @listing = Listing.find(params[:id])
  end

  # POST /listings
  # POST /listings.json
  def create
    @listing = Listing.new(params[:listing])
    @listing.user_id = current_user.id

    respond_to do |format|
      if @listing.save
        format.html { redirect_to @listing, notice: 'Listing was successfully created.' }
        format.json { render json: @listing, status: :created, location: @listing }
      else
        format.html { render action: "new" }
        format.json { render json: @listing.errors, status: :unprocessable_entity }
      end
    end
  end

  # PUT /listings/1
  # PUT /listings/1.json
  def update
    @listing = Listing.find(params[:id])

    respond_to do |format|
      if @listing.update_attributes(params[:listing])
        format.html { redirect_to @listing, notice: 'Listing was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @listing.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /listings/1
  # DELETE /listings/1.json
  def destroy
    @listing = Listing.find(params[:id])
    @listing.destroy

    respond_to do |format|
      format.html { redirect_to listings_url }
      format.json { head :no_content }
    end
  end

  private
  def set_listing
    @listing = Listing.find(params[:id])
  end

  def listing_params
    params.require(:listing).permit(:name, :description, :price, :image)
  end

  def check_user
    if current_user != @listing.user
      redirect_to root_url, alert: "Sorry, this listing belongs to someone else."
    end
  end

end

The code that we had to add for this is the second before_filter and the def check_user

If any other information is needed to help answer this, please let me know.