Can't remove `<% yield p = np %>` from existing partial.

[andrewculver]

Before:

Steps to reproduce:

1. Clone https://github.com/bullet-train-co/bullet_train
2. Get it up and running (e.g. bin/setup, bin/dev, etc.)
3. Run mkdir local; git clone git@github.com:bullet-train-co/nice_partials.git local/nice_partials
4. Add gem "nice_partials", path: "local/nice_partials" to Gemfile
5. Run bin/develop and select the bullet_train-themes-light Ruby gem.
6. Open local/bullet_train-themes-light.
7. Edit app/views/themes/light/_box.html.erb
8. Remove <% yield p = np %>
9. Experience bug.

After:

[andrewculver]

(Also, if I leave <% yield p = np %> in place, everything continues to work fine.)