SAML Group Sync does not work with multiple SAML providers on a self-managed instance

Reported in !76390 (comment 1004141812)

See !76390 (comment 988054489)

Processing by GroupsController#show as HTML
  Parameters: {"id"=>"ANY_GROUP_ID"}
  Rendered layout layouts/group.html.haml (Duration: 495.9ms | Allocations: 192754)
Completed 500 Internal Server Error in 628ms (ActiveRecord: 97.8ms | Elasticsearch: 0.0ms | Allocations: 220486)
  
ActionView::Template::Error (undefined method `[]' for nil:NilClass):
    1: = render partial: 'shared/nav/sidebar', object: Sidebars::Groups::Panel.new(group_sidebar_context(@group, current_user))