ActionView::Template::Error: undefined method `external_wiki_url' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/637608/

NoMethodError: undefined method `external_wiki_url' for nil:NilClass
  /opt/gitlab/embedded/service/gitlab-rails/app/views/layouts/nav/sidebar/_project.html.haml:318:in `_app_views_layouts_nav_sidebar__project_html_haml__1901208262604883293_69813078868180'
    - external_wiki_url = @project.external_wiki.external_wiki_url
  action_view/template.rb:159:in `block in render'
    view.send(method_name, locals, buffer, &block)
  active_support/notifications.rb:166:in `instrument'
    yield payload if block_given?
  action_view/template.rb:354:in `instrument'
    ActiveSupport::Notifications.instrument("!render_template.action_view".freeze, payload, &block)
  action_view/template.rb:157:in `render'
    instrument("!render_template".freeze) do
...
(201 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `external_wiki_url' for nil:NilClass

ActionView::Template::Error: undefined method `external_wiki_url' for nil:NilClass