Error 500 after migrating from 8.5.8 to 8.6.0

I've just run the commands sudo apt-get update and then sudo apt-get install gitlab-ce.

Here is the log file:

...
Started GET "/dashboard/projects/starred" for <IP_masked> at 2016-03-22 18:01:08 +0100
Processing by Dashboard::ProjectsController#starred as HTML
Read fragment views/groups/5-20160304075516896843000/projects/169-20160322164853383591000/projects/starred/application_settings/1-20160314212317749729000/v2.2/86aa46c6c228c49e9392077e9782690a (0.2ms)
Completed 500 Internal Server Error in 325ms (ActiveRecord: 20.5ms)

ActionView::Template::Error (undefined method `main_language' for #<Project:0x0000000649f4e8>):
    28:           = project.name
    29:
    30:     .controls
    31:       - if project.main_language
    32:         %span
    33:           = project.main_language
    34:       - if ci_commit
  app/views/shared/projects/_project.html.haml:31:in `block in _app_views_shared_projects__project_html_haml___1820689258849726490_52189860'
  app/views/shared/projects/_project.html.haml:14:in `_app_views_shared_projects__project_html_haml___1820689258849726490_52189860'
  app/views/shared/projects/_list.html.haml:16:in `block in _app_views_shared_projects__list_html_haml__4256859900705446066_46038840'
  app/views/shared/projects/_list.html.haml:14:in `_app_views_shared_projects__list_html_haml__4256859900705446066_46038840'
  app/views/dashboard/projects/_projects.html.haml:1:in `_app_views_dashboard_projects__projects_html_haml___1095951534482068007_47853380'
  app/views/dashboard/projects/starred.html.haml:10:in `_app_views_dashboard_projects_starred_html_haml___4287266234826350086_53921220'
  app/controllers/dashboard/projects_controller.rb:40:in `starred'
  lib/gitlab/middleware/go.rb:16:in `call'

Related/duplicate of #14541 (closed) and #14347 (closed)