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);

To

$result = "any_string_you_want_here";

Now you can use any key to activate the theme and can install demo content.

UPDATE:

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

return;

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.