Warning: DOMXPath::query(): Invalid expression in /var/www/html/magento2/vendor/magento/framework/Config/Dom.php on line 336

Notice: Trying to get property 'length' of non-object in /var/www/html/magento2/vendor/magento/framework/Config/Dom.php on line 338

Notice: Trying to get property 'length' of non-object in /var/www/html/magento2/vendor/magento/framework/Config/Dom.php on line 345

Fatal error: Uncaught Error: Call to a member function appendChild() on null in /var/www/html/magento2/vendor/magento/framework/Config/Dom.php:218 Stack trace: #0 /var/www/html/magento2/vendor/magento/framework/Config/Dom.php(149): Magento\Framework\Config\Dom->_mergeNode() #1 /var/www/html/magento2/vendor/magento/framework/Config/Reader/Filesystem.php(153): Magento\Framework\Config\Dom->merge() #2 /var/www/html/magento2/vendor/magento/framework/Config/Reader/Filesystem.php(132): Magento\Framework\Config\Reader\Filesystem->_readFiles() #3 /var/www/html/magento2/vendor/magento/framework/App/ObjectManager/ConfigLoader.php(76): Magento\Framework\Config\Reader\Filesystem->read() #4 /var/www/html/magento2/vendor/magento/framework/App/ObjectManager/Environment/Developer.php(79): Magento\Framework\App\ObjectManager\ConfigLoader->load() #5 /var/www/html/magento2/vendor/magento/framework/App/ObjectManagerFactory.php(191): Magento\Framework\App\ObjectManager\Environment\Developer->configureObjectManager() #6 /var/www/html/magento2/ in /var/www/html/magento2/vendor/magento/framework/Config/Dom.php on line 218