I have not been able to solve the following error:

require 'rails_helper'

describe 'Adding todo items' do
  let!(:todo_list) {TodoList.create(title: 'Grocery list', description: 'Grocery' ) }

  def visit_todo_list(list)
    visit '/todo_lists'
    within "#todo_list_#{list.id}" do
      click_link 'List Items'
    end
  end

  it 'Is successful with valid contnt' do
    visit_todo_list(todo_list)
    click_link 'New Todo Item'
    fill_in 'Content', with: 'Milk'
    click_button 'Save'
    expect(page).to have_content('Added todo list item')
    within('ul.todo_items') do
      expect(page).to have_content('Milk')
    end
  end

  it 'displays an error with content less than 2 characters long' do
    visit_todo_list(todo_list)
    click_link 'New Todo Item'
    fill_in 'Content', with: ''
    click_button 'Save'
    within('div.flash') do
      expect(page).to have_content('There was a problem adding that todo list item')
    end
    expect(page).to have_content('Content is too short')
  end
end

class TodoItemsController < ApplicationController
  def index
    @todo_list = TodoList.find(params[:todo_list_id])
  end

  def new
    @todo_list = TodoList.find(params[:todo_list_id])
    @todo_item = @todo_list.todo_items.new
  end

  def create
    @todo_list = TodoList.find(params[:todo_list_id])
    @todo_item = @todo_list.todo_items.new(todo_item_params)
    if @todo_item.save
      flash[:success] = 'Added todo list item'
      redirect_to todo_list_todo_items_path
    else
      flash[:error] = 'There was a problem adding that todo list item'
      render action: :new
    end
  end

  private

  def todo_item_params
    params[:todo_item].permit(:content)
  end
end

class TodoItem < ActiveRecord::Base
  belongs_to :todo_list

  validates :content, presence: true, length: { minimum: 3 }
end