Hello Samuel,
I tried to replicate this on my local setup but I am not getting the any warning message even when my portfolio content is empty.
Also, please note that what you are seeing is just a warning message and not an error message and this is showing up because you debug function is enabled. Having the debug function enabled is helpful when your site is in development or if you are trying to trace an error.
Is it possible to have it disable by when you 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.
You may also 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.
Let us know how it goes.