User returning nil. On line 17 of index.html.erb. Undefined method "username" for nilNilclass

index.html.erb

<%= render 'pages/home' if !user_signed_in? %>

<div id="things" class="transitions-enabled">
  <% @things.each do |thing| %>
    <div class='panel panel default'>
    <div class="box">
      <%= link_to image_tag(thing.image.url(:medium)), thing %>
      <div class='panel-body'>

      <% if thing.link.blank? %>
      <strong><%= thing.title %></strong>
      <% else %>
      <strong><%= link_to thing.title, "http://#{thing.link}"%></strong>
      <% end %>

      <p><%= thing.description %></p>
      By <%= link_to thing.user.username, user_path(thing.user) %>

      <% if thing.user == current_user %>
        <%= link_to edit_thing_path(thing) do %>
        <span class='glyphicon glyphicon-edit'></span>
      <% end %>
      <%= link_to thing_path(thing), method: :delete, data: { confirm: 'Are you sure?' } do %>
        <span class='glyphicon glyphicon-trash'></span>
      <% end %>
      </div>
      <% end %>
      </div>
    </div>
  <% end %>
</div>


<%= will_paginate @posts, renderer: BootstrapPagination::Rails, class: 'pull-left' %>

Log Output

Started GET "/" for 127.0.0.1 at 2013-12-10 20:02:52 -0500
Processing by ThingsController#index as HTML
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 3 ORDER BY "users"."id" ASC LIMIT 1
  Thing Load (0.3ms)  SELECT "things".* FROM "things" ORDER BY created_at DESC LIMIT 50 OFFSET 0
  User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = ? ORDER BY "users"."id" ASC LIMIT 1  [["id", 3]]
  User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = ? ORDER BY "users"."id" ASC LIMIT 1  [["id", 1]]
  Rendered things/index.html.erb within layouts/application (12.7ms)
Completed 500 Internal Server Error in 19ms

ActionView::Template::Error (undefined method `username' for nil:NilClass):
    14:       <% end %>
    15: 
    16:       <p><%= thing.description %></p>
    17:       By <%= link_to thing.user.username, user_path(thing.user) %>
    18: 
    19:       <% if thing.user == current_user %>
    20:         <%= link_to edit_thing_path(thing) do %>
  app/views/things/index.html.erb:17:in `block in _app_views_things_index_html_erb___1232011580397462783_2192440640'
  app/views/things/index.html.erb:4:in `_app_views_things_index_html_erb___1232011580397462783_2192440640'


  Rendered /Users/DylanRichards/.rvm/gems/ruby-2.0.0-p247/gems/actionpack-4.0.1/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.7ms)
  Rendered /Users/DylanRichards/.rvm/gems/ruby-2.0.0-p247/gems/actionpack-4.0.1/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (1.4ms)
  Rendered /Users/DylanRichards/.rvm/gems/ruby-2.0.0-p247/gems/actionpack-4.0.1/lib/action_dispatch/middleware/templates/rescues/template_error.erb within rescues/layout (18.0ms)

show.html.erb

<%= link_to 'Back', :back, class: 'btn btn-xs btn-primary' %>

<div class='row'>
  <div class='col-md-offset-2 col-md-8'>
    <div class='panel panel-default'>
      <div class='panel-heading text-center'>
        <%= image_tag @thing.image.url(:medium) %>
      </div>
      <div class='panel-body'>
        <p>
          <% if @thing.link.blank? %>
          <strong><%= @thing.title %></strong>

          <% else %>
          <strong><%= link_to @thing.title, "http://#{@thing.link}" %></strong>
          <% end %>
        </p>
        <p>
          <%= @thing.description %>
        </p>
        <p>
          By <%= link_to @thing.user.username, @thing.user %>
        </p>
        <% if @thing.user == current_user %>
        <%= link_to edit_thing_path(@thing) do %>
        <span class='glyphicon glyphicon-edit'></span>
        <% end %>
        <% end %>
      </div>
    </div>
  </div>

thing.rb

class Thing < ActiveRecord::Base
  belongs_to :user

  default_scope -> { order('created_at DESC') }

  has_attached_file :image, :styles => { :large => '500x500>', :medium => '300x300>', :thumb => '100x100>' }

  validates :image, presence: true
  validates :title, presence: true, length: { minimum: 5, maximum: 50 }

  # Returns microposts from the users being followed by the given user.
  def self.from_users_followed_by(user)
    followed_user_ids = "SELECT followed_id FROM relationships
                         WHERE follower_id = :user_id"
    where("user_id IN (#{followed_user_ids}) OR user_id = :user_id",
          user_id: user.id)
  end

end

things_controller.rb

class ThingsController < ApplicationController
  before_action :set_thing, only: [:show, :edit, :update, :destroy]
  before_action :authenticate_user!, except: [:index, :show]

  # GET /things
  # GET /things.json
  def index
    @things = Thing.all.order("created_at DESC").paginate(:page => params[:page], :per_page => 50)
  end

  # GET /things/1
  # GET /things/1.json
  def show
  end

  # GET /things/new
  def new
    @thing = current_user.things.build
  end

  # GET /things/1/edit
  def edit
  end

  # POST /things
  # POST /things.json
  def create
    @thing = current_user.things.build(thing_params)

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

  # PATCH/PUT /things/1
  # PATCH/PUT /things/1.json
  def update
    respond_to do |format|
      if @thing.update(thing_params)
        format.html { redirect_to @thing, notice: 'Thing was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @thing.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /things/1
  # DELETE /things/1.json
  def destroy
    @thing.destroy
    respond_to do |format|
      format.html { redirect_to things_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_thing
      @thing = Thing.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def thing_params
      params.require(:thing).permit(:title, :description, :image, :link)
    end
  end

user.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable, #:recoverable,
          :rememberable, :trackable, :validatable

  has_many :things

  validates :name, presence: true, length: { minimum: 2, maximum: 20}
  validates :username, presence: true, length: { minimum: 2, maximum: 20}
  validates :username, uniqueness: true
  validates :email, presence: true
  validates :email, uniqueness: true

  

  has_attached_file :avatar, :styles => { :medium => "300x300>", :thumb => "100x100>" , :nav => "25x25"}

  extend FriendlyId
  friendly_id :username

  def show
    
  end

  #follow features
  has_many :followed_users, through: :relationships, source: :followed
  has_many :relationships, foreign_key: "follower_id", dependent: :destroy
  has_many :followed_users, through: :relationships, source: :followed

  def following?(other_user)
    relationships.find_by(followed_id: other_user.id)
  end

  def follow!(other_user)
    relationships.create!(followed_id: other_user.id)
  end

  def unfollow!(other_user)
    relationships.find_by(followed_id: other_user.id).destroy!
  end

  def feed
    Thing.from_users_followed_by(self)
  end

  has_many :reverse_relationships, foreign_key: "followed_id",
                                   class_name:  "Relationship",
                                   dependent:   :destroy
  has_many :followers, through: :reverse_relationships, source: :follower


end

users_controller.rb

class UsersController < ApplicationController
  def show
    @user = User.find_by_username(params[:id])
  end

  def user_params
      params.require(:user).permit(:avatar)
  end


  def following
      @title = "Following"
      @user = User.find_by_username(params[:id])
      @users = @user.followed_users.paginate(page: params[:page])
      render 'show_follow'
    end

    def followers
      @title = "Followers"
      @user = User.find_by_username(params[:id])
      @users = @user.followers.paginate(page: params[:page])
      render 'show_follow'
    end
end