Can’t create new pages anymore

Hi,

Recently updated to WP4.8 and now i can’t publish new pages, i receive this error message:
Notice: Trying to get property of non-object in /var/www/vhosts/vps-281860-5378.hosted.at.hostnet.nl/blitzbrokers.com/wp-includes/nav-menu.php on line 728
Are you sure you want to do this?

Please try again.

I switched off all plugins and changed to twenty thirteen theme but problem remained.
I checked the line in the nav_menu.php file nr 728 and this was correct.

Does anybody know what to do?

Thanks in advance