【Rails5】エラー：undefined method `url' for nil:NilClassの解決方法を教えてください。

<div class="card">
    <% @how_to_makes.each do |process| %>
      <ol>
        <li>
          <%= image_tag process.process_image.url, class: "card-img-top" %>
          <div class="card-body"><%= process.explanation %></div>
        </li>
      </ol>
    <% end %>
  </div>

class Recipe < ApplicationRecord
  belongs_to :user, optional: true
  has_many :recipe_ingredients, dependent: :destroy
  has_many :how_to_makes, dependent: :destroy
  accepts_nested_attributes_for :recipe_ingredients, allow_destroy: true
  accepts_nested_attributes_for :how_to_makes, allow_destroy: true

  mount_uploader :image, ImageUploader
  mount_uploader :process_image, ProcessImageUploader
end

class CreateHowToMakes < ActiveRecord::Migration[5.2]
  def change
    create_table :how_to_makes do |t|
      t.references :recipe, foreign_key: true
      t.text :explanation
      t.string :process_image
      t.integer :order_no

      t.timestamps
    end
  end
end

class RecipesController < ApplicationController
  before_action :require_user_logged_in, only: [:new, :create, :edit, :update, :destroy]

  def show
    @recipe = Recipe.find(params[:id])
    @recipe_ingredients = @recipe.recipe_ingredients.all
    @how_to_makes = @recipe.how_to_makes.all
  end

  def new
    @recipe = Recipe.new
    @recipe_ingredients = @recipe.recipe_ingredients.build
    @how_to_makes = @recipe.how_to_makes.build
  end

  def create
    @recipe = Recipe.new(recipe_params)
    if @recipe.save
      flash[:success] = 'レシピを投稿しました。'
      redirect_to root_url
    else
      flash.now[:danger] = 'レシピの投稿に失敗しました。'
      render :new
    end
  end


  private

  def recipe_params
    params.require(:recipe).permit(:title, :catchcopy, :no_of_dish, :image, 
                                  recipe_ingredients_attributes:[:id, :recipe_id, :ing_name, :quantity, :_destroy],
                                  how_to_makes_attributes:[:id, :explanation, :process_image, :order_no, :_destroy])
  end
end

<div class="recipe-post">
  <%= form_with(model: @recipe, local: true) do |f| %>



    <div class="form-group mt-2">
      <%= f.fields_for :how_to_makes do |k| %>
        <%= render "recipes/how_to_make_fields", f: k %>
      <% end %>
    </div>

    <div id="detail-association-insertion-point2"></div>


    <div class="col-10 mx-auto">
      <%= link_to_add_association "+フォームを追加", f, :how_to_makes,
      class: "btn btn-secondary btn-block",
      data: {
      association_insertion_node: '#detail-association-insertion-point2',
      association_insertion_method: 'after'
      }%>
    </div>
    <div class="col-10 mx-auto">
      <%= f.submit 'レシピ投稿！', class: 'mt-2 btn btn-secondary btn-block' %>
    </div>
  <% end %>
</div>

<div class="nested-fields">
  <div class="row">
    <div class="col-5"><%= f.text_area :explanation, class: "form-control", placeholder: "説明" %></div>
    <div><%= f.file_field :process_image %></div>
    <div class="col-1 px-0 w-auto">
      <%= link_to_remove_association "削除", f, class: "btn btn-secondary btn-block" %>
    </div>
  </div>
</div>