Fix preview submission (no sesskey)

Author: laurentdavid

classes/output/renderer.php

@@ -378,6 +378,7 @@ public function print_preview_formend($url, $submitstr, $resetstr) {
         $output .= \html_writer::start_tag('div');
         $output .= \html_writer::empty_tag('input', ['type' => 'submit', 'name' => 'submit', 'value' => $submitstr,
             'class' => 'btn btn-primary']);
+        $output .= \html_writer::empty_tag('input', ['type' => 'hidden', 'name' => 'sesskey', 'value' => sesskey()]);
         $output .= ' ';
         $output .= \html_writer::tag('a', $resetstr, ['href' => $url, 'class' => 'btn btn-secondary mr-1']);
         $output .= \html_writer::end_tag('div') . "\n";