`save' for nil:NilClass

Failure/Error: click_link "List Items"
     ActionController::RoutingError:
       undefined method `save' for nil:NilClass
     # ./app/controllers/todo_items_controller.rb:16:in `<class:TodoItemsController>'
     # ./app/controllers/todo_items_controller.rb:1:in `<top (required)>'
     # ./spec/features/todo_items/create_spec.rb:9:in `block in visit_todo_list'
     # ./spec/features/todo_items/create_spec.rb:8:in `visit_todo_list'
     # ./spec/features/todo_items/create_spec.rb:14:in `block (2 levels) in <top (required)>'

require 'spec_helper'

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

  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 content" do
    visit_todo_list(todo_list)
    click_link "New Todo Item"
    fill_in "Content", with: "Milk"
    click_button "Save"
    expect(page).to have_content("Adding tdo list item.")
    within("ul.todo_items") do
      expect(page). to have_content("Milk")
    end
  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)
  end

    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


    private

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