Fixed error in exercise selection in main exercises list - loosely refs BT#6718

main/exercice/exercice.php

@@ -698,7 +698,7 @@ if (!empty($exercise_list)) {
                         }*/
                     }
 
-                    if ($app['security']->isGranted('ROLE_SESSION_MANAGER')) {
+                    if ($app['security']->isGranted('ROLE_SESSION_MANAGER') && !empty($exercise_id)) {
                         $actions .= Display::url(
                             Display::return_icon('admin_star.png', get_lang('Distribution'), '', ICON_SIZE_SMALL),
                             $app['url_generator']->generate('exercise_distribution.controller:indexAction', array('exerciseId' => $exercise_id))