DOMDocument::load(): I/O warning : failed to load external entity - on PHP 7.4 when entering the themes screen

Seems some new functions were added which call internally the libxml extension.
On PHP 7.4 (and libxml before 2.9.0), when that is used, without disabling the loading of external entities, the script fails

Adding the following before $domDocument->load($configFile), make things work.

                if (\PHP_VERSION_ID < 80000) {
                    libxml_disable_entity_loader(false); // @see: http://phpsecurity.readthedocs.io/en/latest/Injection-Attacks.html#xml-external-entity-injection
                }