Magento get error after run setup:upgrade

[annq3sivn4281]

I have this issue when i run the setup:upgrade command, can anybody explain to me what is this error and how to fix it:

DOMDocument::loadXML(): Argument #1 ($source) must not be empty#0 /var/www/html/rakuten_magento/vendor/magento/framework/Config/Dom.php(451): DOMDocument->loadXML()
#1 /var/www/html/rakuten_magento/vendor/magento/framework/Config/Dom.php(148): Magento\Framework\Config\Dom->_initDom()
#2 /var/www/html/rakuten_magento/vendor/magento/framework/Config/Reader/Filesystem.php(153): Magento\Framework\Config\Dom->merge()
#3 /var/www/html/rakuten_magento/vendor/magento/framework/Config/Reader/Filesystem.php(132): Magento\Framework\Config\Reader\Filesystem->_readFiles()
#4 /var/www/html/rakuten_magento/vendor/magento/framework/Setup/Declaration/Schema/Declaration/ReaderComposite.php(46): Magento\Framework\Config\Reader\Filesystem->read()
#5 /var/www/html/rakuten_magento/vendor/magento/framework/Setup/Declaration/Schema/SchemaConfig.php(76): Magento\Framework\Setup\Declaration\Schema\Declaration\ReaderComposite->read()
#6 /var/www/html/rakuten_magento/setup/src/Magento/Setup/Model/DeclarationInstaller.php(57): Magento\Framework\Setup\Declaration\Schema\SchemaConfig->getDeclarationConfig()
#7 /var/www/html/rakuten_magento/setup/src/Magento/Setup/Model/Installer.php(846): Magento\Setup\Model\DeclarationInstaller->installSchema()
#8 /var/www/html/rakuten_magento/setup/src/Magento/Setup/Model/Installer.php(894): Magento\Setup\Model\Installer->declarativeInstallSchema()
#9 /var/www/html/rakuten_magento/setup/src/Magento/Setup/Console/Command/UpgradeCommand.php(146): Magento\Setup\Model\Installer->installSchema()
#10 /var/www/html/rakuten_magento/vendor/symfony/console/Command/Command.php(255): Magento\Setup\Console\Command\UpgradeCommand->execute()
#11 /var/www/html/rakuten_magento/vendor/symfony/console/Application.php(1009): Symfony\Component\Console\Command\Command->run()
#12 /var/www/html/rakuten_magento/vendor/symfony/console/Application.php(273): Symfony\Component\Console\Application->doRunCommand()
#13 /var/www/html/rakuten_magento/vendor/magento/framework/Console/Cli.php(115): Symfony\Component\Console\Application->doRun()
#14 /var/www/html/rakuten_magento/vendor/symfony/console/Application.php(149): Magento\Framework\Console\Cli->doRun()
#15 /var/www/html/rakuten_magento/bin/magento(23): Symfony\Component\Console\Application->run()
#16 {main}

[Mukesh Tiwari]

Hi @annq3sivn4281 

You may have some empty xml file Or the xml may not be valid.

Following is the function triggering the error.

   protected function _initDom($xml)
    {
        $dom = new \DOMDocument();
        $useErrors = libxml_use_internal_errors(true);
        $res = $dom->loadXML($xml); // Line 451

[annq3sivn4281]

Where can i find this function ?