getHTML() returns calls __clone method called on non-object

[manzari]

Steps to reproduce:

$inlineStyleBody = new InlineStyle($body);
$cssDir = new DirectoryIterator("./css");
    foreach ($cssDir as $cssFile) {
        if ($cssFile->isDot() or $cssFile->getExtension() != 'css') {
            continue;
         };
         $inlineStyleBody->applyStylesheet($cssFile->getPathname());
    }
    file_put_contents('testout.html', $inlineStyleBody->getHTML())

Expected behavior:
Create HTML-Document with inlined css

Actual behavior:
Returning following error:

Fatal error: Uncaught Error: __clone method called on non-object in /var/www/html/vendor/inlinestyle/inlinestyle/InlineStyle/InlineStyle.php on line 397
Error: __clone method called on non-object in /var/www/html/vendor/inlinestyle/inlinestyle/InlineStyle/InlineStyle.php on line 397

Call Stack:
    [...]
    0.2193    2416360   4. InlineStyle\InlineStyle->getHTML() /var/www/html/src/MyMail.php:91
    0.2193    2416416   5. InlineStyle\InlineStyle->__clone() /var/www/html/vendor/inlinestyle/inlinestyle/InlineStyle/InlineStyle.php:180

Process finished with exit code 255