Hello James,
This should be a warning message caused by a third party plugin so you can try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
It is also possible to simple get rid of this message by disabling your WP_DEBUG. You can login through FTP then edit the wp-config.php file and find the line: define( 'WP_DEBUG', true );. Set it to false then save the file.
Hope this helps.