Get All Job Listing Fields

I am trying to get a list (array) of all the job listing fields that have been added using the filter job_manager_job_listing_data_fields. The code I am using is:

$fields = new WP_Job_Manager_Writepanels();
$fields = $fields->job_listing_fields();

This just gives the following error:

Class ‘WP_Job_Manager_Writepanels’ not found

If this is because this class is only included in the admin. Is there another way of getting an array of all the fields added to that filter please.

Many thanks.

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