NoMethodError (undefined method `comments' for nil:NilClass):

app/controllers/comments_controller.rb:9:in `create'

class CreateComments < ActiveRecord::Migration[5.1]
  def change
    create_table :comments do |t|
      t.references :user, foreign_key: true
      t.text :body, null:false
      t.references :commentable, polymorphic: true, index: true

      t.timestamps
    end
      add_index :comments, [:commentable_id, :commentable_type]
  end
end

Rails.application.routes.draw do

  resources :comments, only: [:create]

  resources :votes, only: [:create] do
    delete :reset, on: :collection
  end

  resources :attachments, only: :destroy

  devise_for :users

  resources :questions do
    resources :comments, only: :create, defaults: { commentable: 'questions' }
    resources :answers, shallow: true do
      resources :comments, only: :create, defaults: { commentable: 'answers' }
      patch :set_best, on: :member
    end
  end

  root to: "questions#index"

  mount ActionCable.server => '/cable'
end

resources :attachments, only: :destroy

      devise_for :users

      resources :questions do
        resources :comments, only: :create, defaults: { commentable: 'questions' }
        resources :answers, shallow: true do
          resources :comments, only: :create, defaults: { commentable: 'answers' }
          patch :set_best, on: :member
        end
      end

      root to: "questions#index"

      mount ActionCable.server => '/cable'
    end


 comments form partial _comment_form.html.slim:

    h4 Add a comment
    .comment_errors
    = form_for [@commentable, Comment.new], remote: true do |f|
      .form-group
        = f.label :body, 'Comment'
        = f.text_area :body, class: 'form-control'
      p= f.submit 'Create', class: 'btn btn-primary'


class CommentsController < ApplicationController
  before_action :authenticate_user!
  before_action :load_commentable, only: [:create]

  def create
    @comment = @commentable.comments.create(comment_params.merge(user: current_user))
  end

  private

  def load_commentable
    if params[:comment][:commentable_type] == 'Answer'
      @commentable = Answer.find(params[:comment][:commentable_id])
      gon.answer_id = @commentable.id
    elsif params[:comment][:commentable_type] == 'Question'
      gon.answer_id = @commentable.id
      @commentable = Question.find(params[:comment][:commentable_id])
    end
  end

  def comment_params
    params.require(:comment).permit(:body, :commentable_type, :commentable_id )
  end
end

h3 Comments
.question_comments id="comments_section_question_#{@question.id}"
  - if user_signed_in?
    .row
      = render 'comments/comment_form', commentable: @question