http500 when 'add to mattermost' error:undefined method `each' for nil:NilClass

Summary

error log

==> /var/log/gitlab/gitlab-rails/production.log <==
Started GET "/xxxxxxxxxxx/xxxxxxxx/mattermost/new" for 127.0.0.1 at 2019-03-22 06:42:20 +0000
Processing by Projects::MattermostsController#new as HTML
  Parameters: {"namespace_id"=>"xxxxxxxxx", "project_id"=>"xxxxxxxxx"}
Completed 500 Internal Server Error in 102ms (ActiveRecord: 7.5ms)
  
NoMethodError (undefined method `each' for nil:NilClass):
  
lib/mattermost/session.rb:184:in `parse_cookie'
lib/mattermost/session.rb:131:in `oauth_uri'
lib/mattermost/session.rb:103:in `create'
lib/mattermost/session.rb:39:in `block in with_session'
lib/mattermost/session.rb:159:in `with_lease'
lib/mattermost/session.rb:38:in `with_session'
lib/mattermost/client.rb:14:in `with_session'
lib/mattermost/client.rb:34:in `session_get'
lib/mattermost/team.rb:7:in `all'
app/models/project_services/mattermost_slash_commands_service.rb:34:in `list_teams'
app/controllers/projects/mattermosts_controller.rb:37:in `teams'
lib/gitlab/i18n.rb:55:in `with_locale'
lib/gitlab/i18n.rb:61:in `with_user_locale'
app/controllers/application_controller.rb:423:in `set_locale'
lib/gitlab/middleware/multipart.rb:103:in `call'
lib/gitlab/request_profiler/middleware.rb:16:in `call'
lib/gitlab/middleware/go.rb:20:in `call'
lib/gitlab/etag_caching/middleware.rb:13:in `call'
lib/gitlab/middleware/correlation_id.rb:16:in `block in call'
lib/gitlab/correlation_id.rb:15:in `use_id'
lib/gitlab/middleware/correlation_id.rb:15:in `call'
lib/gitlab/middleware/read_only/controller.rb:40:in `call'
lib/gitlab/middleware/read_only.rb:18:in `call'
lib/gitlab/middleware/basic_health_check.rb:25:in `call'
lib/gitlab/request_context.rb:20:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:29:in `call'
lib/gitlab/middleware/release_env.rb:13:in `call'

==> /var/log/gitlab/gitlab-rails/production_json.log <==

Env

System : ubuntu 16.04 Docker : 18.09.0 Gitlab : CE 11.8.3 (gitlab-ce@3f8131133bf6310a7fc05d7a332b4560edb456ef)