prevent php8-warnings, closes #3476

Closes #3476

Merge request studip/studip!2372

app/controllers/calendar/schedule.php

@@ -243,7 +243,11 @@ class Calendar_ScheduleController extends AuthenticatedController
                 $this->render_template('calendar/schedule/_entry_course');
             } else if ($id) {
                 $entry_columns = CalendarScheduleModel::getScheduleEntries($GLOBALS['user']->id, 0, 0, $id);
-                $entries = array_pop($entry_columns)->getEntries();
+                $entries = [];
+                $entry_columns = array_pop($entry_columns);
+                if ($entry_columns) {
+                    $entries = $entry_columns->getEntries();
+                }
                 $this->show_entry = array_pop($entries);
                 $this->render_template('calendar/schedule/_entry_schedule');
             }