When deleting a product, it gives an error, how to solve?

Question

Ruby on Rails

I am engaged in the book "rails 4 agile web application development" in chapter 11, I ran into a problem.
when empty cart is clicked in the generated left menu:
Showing /depot/app/views/layouts/application.html.erb where line #17 raised:
'nil' is not an ActiveModel-compatible object. It must implement :to_partial_path.
complains about :
<%= render @cart %>
before chapter 11 edits everything worked.
I've been suffering for two days now :(

Answer 1

'nil' is not an ActiveModel-compatible object.

What do you think this phrase means?

Answer 2

there are just suspicions that the book was silent about this error, because after 20 pages it fixes this error, only related to the /products section

Answer 3

<%= render @cart if @cart %>