Answered accessing extended user object in getlist processor

I am trying to modify the superbox select plugin to query a list of my extended user class (extUser) which simply adds a relation to a custom table to the user class.

If I switch the 'classname' and 'displayfield' to the custom object I relate the users to, I can get a list of my custom objects just fine.

If I try to switch the classname to 'extUser' (using username as a display field) I get no results.

Here is a simple version of the getlist processor class.

Can anyone tell me what I am missing?

class SuperboxselectResourcesGetListProcessor extends modObjectGetListProcessor {
    public $classKey = 'extUser';
    public $languageTopics = array('superboxselect:default');
    public $defaultSortField = 'username';
    public $defaultSortDirection = 'DESC';
    public $objectType = 'resellers.extuser';
    
    public $displayField;
    
    public function prepareQueryBeforeCount(xPDOQuery $c) {
        
        // Get Properties
        $parents = explode(',', $this->getProperty('parents', '0'));
        $classKey = $this->getProperty('className', 'extUser');
        //$modx->log(3,'class key: ' . $classKey);
        $displayField = $this->getProperty('displayField', 'username');
        $this->displayField = $displayField;
        $resource_id = intval($this->getProperty('resource_id'));
        $context_key = $this->getProperty('context_key', false);
        $depth = $this->getProperty('depth', 10);
        $limitRelatedContext = $this->getProperty('limitRelatedContext', false);
        
        // Get the context of the current edited resource
        if (!$context_key) {
            $resource = $this->modx->getObject('modResource', $resource_id);
            $context_key = $resource->get('context_key');
        }
        
        $c->select(array('id', $displayField));
        
        $c->where(array(
            'deleted' => false,
            'published' => true
        ));
        return $c;
    }
    
    public function prepareQueryAfterCount(xPDOQuery $c) {
        $displayField = $this->getProperty('displayField', 'username');
        $c->sortby($displayField, 'ASC');
        return $c;
    }
}

return 'SuperboxselectResourcesGetListProcessor';