ActionView::Template::Error: undefined method `can?' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/4461/

NoMethodError: undefined method `can?' for nil:NilClass
  app/views/projects/project_members/_shared_group_members.html.haml:11:in `block in _app_views_projects_project_members__shared_group_members_html_haml__2399435408425158750_70315180915700'
    - if current_user.can?(:admin_group, shared_group)
  app/views/projects/project_members/_shared_group_members.html.haml:1:in `_app_views_projects_project_members__shared_group_members_html_haml__2399435408425158750_70315180915700'
    - @project_group_links.each do |group_links|
  app/views/projects/project_members/index.html.haml:27:in `_app_views_projects_project_members_index_html_haml__1470426806697583315_144350940'
    = render "shared_group_members"
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
...
(145 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `can?' for nil:NilClass
  app/views/projects/project_members/_shared_group_members.html.haml:11:in `block in _app_views_projects_project_members__shared_group_members_html_haml__2399435408425158750_70315180915700'
    - if current_user.can?(:admin_group, shared_group)
  app/views/projects/project_members/_shared_group_members.html.haml:1:in `_app_views_projects_project_members__shared_group_members_html_haml__2399435408425158750_70315180915700'
    - @project_group_links.each do |group_links|
  app/views/projects/project_members/index.html.haml:27:in `_app_views_projects_project_members_index_html_haml__1470426806697583315_144350940'
    = render "shared_group_members"
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
...
(145 additional frame(s) were not displayed)

URL that triggers this: https://gitlab.com/gnutls/gnutls/project_members (visit this when logged out)

cc @rspeicher @DouweM