prevent warning, fixes #2056

Closes #2056

Merge request studip/studip!1337

templates/header.php

@@ -258,7 +258,12 @@ if ($navigation) {
 
     <div id="current-page-structure" <? if (!($contextable)) echo 'class="contextless"'; ?>>
 
-        <? if (PageLayout::isHeaderEnabled() && Navigation::hasItem('/course') && Navigation::getItem('/course')->isActive() && $_SESSION['seminar_change_view_'.Context::getId()]) : ?>
+        <? if (
+            PageLayout::isHeaderEnabled()
+            && Navigation::hasItem('/course')
+            && Navigation::getItem('/course')->isActive()
+            && !empty($_SESSION['seminar_change_view_'.Context::getId()])
+        ) : ?>
             <?= $this->render_partial('change_view', ['changed_status' => $_SESSION['seminar_change_view_'.Context::getId()]]) ?>
         <? endif ?>