error when i click on follow button ... Relationship is not working

All i am trying to create follow and follwers relationship and i am experiencing some issues. Chris i think you should create a course on this as well. i am getting issue undefined method `id' for nil:NilClass

Secondly i created a follow button class and that not being displayed on the page when i render it

Relationship.rb

class Relationship < ActiveRecord::Base

  belongs_to :follower, class_name: "User"
  belongs_to :followed, class_name: "User"

  validates :follower_id, :presence => true
  validates :followed_id, :presence => true

  validate :validate_followers

  def validate_followers
    errors.add(:follower_id, "You cannot follow yourself") if follower_id == followed_id
  end
end

User.rb

class User < ActiveRecord::Base
 mount_uploader :avatar, AvatarUploader
  mount_uploader :campaignimage, ImageUploader

  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
acts_as_follower
acts_as_followable

         # User Avatar Validation
  validates_integrity_of  :avatar
  validates_processing_of :avatar
 validates_integrity_of  :campaignimage
  validates_processing_of :campaignimage
   validates :first_name,:last_name,  presence: true  



    has_many :createcampaigns, dependent: :destroy
    has_many :relationships, foreign_key: "follower_id", dependent: :destroy
    has_many :squeals, dependent: :destroy
    has_many :tweets
    has_many :comments, as: :commentable
    has_many:reviews, dependent: :destroy
    has_many:createcampaign, dependent: :destroy
    has_many :posts, dependent: :destroy

     has_many :posts, dependent: :destroy # remove a user's posts if his account is deleted.
         has_many :active_relationships, class_name: "Relationship", foreign_key: "follower_id", dependent: :destroy
         has_many :passive_relationships, class_name: "Relationship", foreign_key: "followed_id", dependent: :destroy

         has_many :following, through: :active_relationships, source: :followed
         has_many :followers, through: :passive_relationships, source: :follower





     # follow another user
         def follow(other)
           active_relationships.create(followed_id: other.id)
         end

         # unfollow a user
         def unfollow(other)
           active_relationships.find_by(followed_id: other.id).destroy
         end

         # is following a user?
         def following?(other)
           following.include?(other)
         end



   private
    def avatar_size_validation
      errors[:avatar] << "should be less than 500KB" if avatar.size > 0.5.megabytes
    end

end

Users.rb

devise_for :users 
  resources :users do

    member do
     get :following, :followers
    end
    resources :createcampaigns
  resources :comments,module: :user
  end

user controller

class UsersController < ApplicationController
 before_action :authenticate_user,
                only: [:index, :edit, :update, :destroy, :following, :followers]

  def show

    if (User.find_by_username(params[:id]))
      @username = params[:id]

    else 
      # redirect to 404 (root for now)
      redirect_to root_path, :notice=> "User not found!" 
    end
    @user = User.find_by_username(params[:username])

     #@user = current_user
    @username= User.find_by_username(params[:username])
    @currentuser = current_user
     @allcampaign = Createcampaign.all
      @toFollow = User.all.last(5)
  end


  def index
  end



  def new
  @user = User.new

end


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

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


end 

_follow_form

  <div id="follow_form">
  <% if current_user.following?(@user) %>
    <%= render 'unfollow' %>
  <% else %>
    <%= render 'follow' %>
  <% end %>
  </div>

when i click on follow: i get the following

undefined method `id' for nil:NilClass

Extracted source (around line #43):
41
42
43
44
45
46

 # follow another user
     def follow(other)
       active_relationships.create(followed_id: other.id)
     end

     # unfollow a user


             Another class is not being displayed when i render it in the page

             <% if current_user.username != @user.username %>
where is it

            <% if !current_user.following?(@user) %>
                <%= form_for(current_user.active_relationships.build) do |f| %>
                    <div><%= hidden_field_tag :followed_id, user.id %></div>
                    <%= f.submit "Follow", class: "btn btn-primary" %>
                <% end %>
            <% else %>
                <%= form_for(current_user.active_relationships.find_by(followed_id: user.id),
                    html: { method: :delete }) do |f| %>
                    <%= f.submit "Unfollow", class: "btn" %>
                <% end %>
            <% end %>

<% end %>