The theme can work without nulling but below step is necessary to order to install demo content.
In the file wp-content\themes\jnews\class\Util\ValidateLicense.php, change line 193 from:
$result = $this->check_purchase_wordpress_themes($token);
$result = "any_string_you_want_here";
Now you can use any key to activate the theme and can install demo content.
The theme checks for updates (the missing slug is part of the data that is stored in the database when you do a legit activation – the theme checks your purchase details with the Envato API and store the returned data… as we don’t have a legit purchase, this data isn’t stored!)
The easiest way to deal with the error is to find the function update_themes in the jnews/class/Util/ValidateLicense.php file and add as the first line of the function after the opening curly bracket
That should stop the update check from running.
Or comment out lines 48 and 49 which are
add_filter('pre_set_site_transient_update_themes', array($this, 'update_themes')); add_filter('pre_set_transient_update_themes', array($this, 'update_themes'));
which will stop the function from being called in the first place.