plugin wordpress seo yoast gives fatal error on line 257

Hi, i had installed wordpress seo yoast version 1.2.3.
After i did this and i went to the website https://www.vaessencopacking.nl i saw a fatal error message:

Fatal error: Call to undefined function is_admin_bar_showing() in /home/vaessencopa/domains/vaessencopacking.nl/public_html/wp-content/plugins/wordpress-seo/inc/wpseo-non-ajax-functions.php on line 257

I looked it up and this is the code there:

function wpseo_admin_bar_css() {
if ( is_admin_bar_showing() )
wp_enqueue_style( ‘boxes’, WPSEO_URL.’css/adminbar.css’, WPSEO_VERSION );
}

I don’t have a clue what’s going wrong, can anybody help me what to do?? I have deactived the plugin for now.

Thanks a lot!!!
Greetings Gaby