Deleting polymorphic associations

I have implemented a favoriting system for my rails app by following this great article.

Here is my set up:

---

favorite.rb

class Favorite < ActiveRecord::Base
  belongs_to :user
  belongs_to :favorited, polymorphic: true
end

---

project.rb

class Project < ActiveRecord::Base
    belongs_to :user

    has_many :favorites, as: :favorited
    has_many :fans, through: :favorites, source: :user 

---

user.rb

class User < ActiveRecord::Base
  has_many :listings
  has_many :projects
  has_many :favorites
  has_many :favorite_listings, through: :favorites, source: :favorited, source_type: 'Listing'
  has_many :favorite_projects, through: :favorites, source: :favorited, source_type: 'Project'

---

favorite_projects_controller.rb

class FavoriteProjectsController < ApplicationController
  before_action :set_project
  # before_action :correct_user
  # before_action :authenticate_user!

  def create
    if Favorite.create(favorited: @project, user: current_user)
      redirect_to @project, notice: 'Project has been favorited'
    else
      redirect_to @project, alert: 'Something went wrong...*sad panda*'
    end
  end

  def destroy
    Favorite.where(favorited_id: @project.id, user_id: current_user.id).first.destroy
    redirect_to @project, notice: 'Project is no longer in favorites'
  end

  private

  def set_project
    @project = Project.find(params[:project_id] || params[:id])
  end
end

---

Here is the problem.

I had some favorited projects.

I used Project.delete_all to delete my projects while certain projects were still "favorited" but now I get an error:

 ActionView::Template::Error (undefined method `favorite_projects' for nil:NilClass):

I'm sure If I had "unfavorited" all those projects before deleting them, this error would not come up.

Does anyone have an idea on how to fix this?