Display related pages under the related category

$url = $modx->makeUrl($doc->get('id'), "", "", "full");

$output .= '<h2><strong><a href="' . $url . '">' . $resource->get('pagetitle') . '</a></strong></h2>' .  $resource->get('content');

<?php
$tvId = 30; /* change this to the ID of the category TV */
  
function getDocs($category, $tvId) {
    global $modx;
    $tvrs = $modx->getCollection('modTemplateVarResource', array(
        'tmplvarid' => $tvId,
        'value'     => $category
    ));
    $docs = array();
    foreach ($tvrs as $tvr) {
        $docs[] = $tvr->get('contentid');
    }
  
    $c = $modx->newQuery('modResource');
    $c->where(
      array('id:in' => $docs)
    );
    $c->sortby('menuindex', 'ASC'); 

 
  
    $resources = $modx->getCollection('modResource', $c);

    $url = $modx->makeUrl($doc->get('id'), "", "", "full");

 
    $output = '<div class="someclasses"><div class="someclasses2"><p>' . $category . '</p>';
 
    foreach ($resources as $resource) {
 
 
/*start formatting output */
$output .= '<h2><strong><a href="' . $url . '">' . $resource->get('pagetitle') . '</a></strong></h2>' .  $resource->get('content');
 
/*end formatting output */
 
    }


$output .= '</div></div>';
  
    return $output;
}
  
/* Get all possible categories */
$c = $modx->newQuery('modTemplateVarResource');
$c->distinct();
$c->where(array('tmplvarid' => $tvId));
  
$tvrs = $modx->getCollection('modTemplateVarResource', $c);
  
$output = '';
/* Get the resources in each category */
$usedCategories = array();
foreach ($tvrs as $tvr) {
    $category = $tvr->get('value');
    if (! in_array($category, $usedCategories)) {
        $usedCategories[] = $category;
        $output .= getDocs($category, $tvId);
       }
    }
  
return $output;

 Fatal error: Call to a member function get() on a non-object in C:\wamp\www\nestnew\ore\cache\includes\elements\modsnippet\46.include.cache.php on line 28

$url = $modx->makeUrl($resource->get('id'), "", "", "full");

$c->sortby('menuindex', 'ASC');