FIX: Avoid validation error when deleting users with locked trust level

app/models/group_user.rb

@@ -60,6 +60,7 @@ class GroupUser < ActiveRecord::Base
 
   def recalculate_trust_level
     return if group.grant_trust_level.nil?
+    return if self.destroyed_by_association&.active_record == User # User is being destroyed, so don't try to recalculate
 
     Promotion.recalculate(user)
   end