Display error if non existant conference's versions are queried

app/controllers/admin/versions_controller.rb

@@ -8,9 +8,12 @@ module Admin
       @conference_id = params[:conference_id].to_i unless params[:conference_id].nil?
       if @conference_id.nil?
         @versions = PaperTrail::Version.where(["conference_id IN (?) OR item_type = 'User'", @conf_ids_for_organizer])
+      elsif !Conference.exists?(id: @conference_id)
+        redirect_to admin_revision_history_path, error: "Conference with ID #{@conference_id} does not exist!"
+        return
       else
-        @versions = PaperTrail::Version.where(['conference_id IN (?)', (@conf_ids_for_organizer & [@conference_id])]).
-                                                                    where.not(item_type: 'User')
+        authorize! :index, PaperTrail::Version.new(conference_id: @conference_id)
+        @versions = PaperTrail::Version.where(conference_id:  @conference_id)
       end
     end
 

app/views/admin/versions/index.html.haml

@@ -4,12 +4,7 @@
 
       .dropdown.pull-right
         %button.btn.btn-success.dropdown-toggle{'data-toggle' => 'dropdown', type: 'button'}
-          - if @conference_id.nil?
-            All Conferences & Users
-          - elsif @conf_ids_for_organizer.include?(@conference_id)
-            = Conference.find(@conference_id).short_title
-          - else
-            Filter By Conference
+          = @conference_id.nil? ? 'All Conferences & Users' : Conference.find(@conference_id).short_title
           %span.caret
         %ul.dropdown-menu
           %li= link_to 'All Conferences & Users',  admin_revision_history_path