Fix core not being able to write to session when making tokens

This would cause a mismatch between tokens generated during H5P
integration setup and the token checks when posting answers in embeds

embed.php

@@ -71,9 +71,6 @@ try {
 $context = context_module::instance($cm->id);
 require_capability('mod/hvp:view', $context);
 
-// Unlock session during embed.
-core\session\manager::write_close();
-
 // Set up view assets.
 $view = new \mod_hvp\view_assets($cm, $course, [
     'disabledownload'   => $disabledownload,
@@ -82,6 +79,9 @@ $view = new \mod_hvp\view_assets($cm, $course, [
 $content = $view->getcontent();
 $view->validatecontent();
 
+// Release session while loading the rest of our assets.
+core\session\manager::write_close();
+
 // Configure page.
 $PAGE->set_url(new \moodle_url('/mod/hvp/embed.php', array('id' => $id)));
 $PAGE->set_title(format_string($content['title']));