Hello,
What version of PHP are you using? PHP 5.4 or previous?
Although we recommend you to update to a newer version of PHP (5.6 or 7), to fix it with old versions of PHP, please edit classes/TransformDocAdvHTML.php and replace that line (3099):
if ($this->complexField !== null && $this->complexField['type'] == 'FORMTEXT' && !empty(trim($childNode->nodeValue))) {
by:
$childNodeValue = trim($childNode->nodeValue);
if ($this->complexField !== null && $this->complexField['type'] == 'FORMTEXT' && !empty($childNodeValue)) {
Regards.