<?php $_menu = $this->renderCategoriesMenuHtml(0,'level-top') ?>

public function renderCategoriesMenuHtml($level = 0, $outermostItemClass = '', $childrenWrapClass = '')
{
    $activeCategories = array();
    foreach ($this->getStoreCategories() as $child) {
        if ($child->getIsActive()) {
            $activeCategories[] = $child;
        }
    }
    $activeCategoriesCount = count($activeCategories);
    $hasActiveCategoriesCount = ($activeCategoriesCount > 0);

    if (!$hasActiveCategoriesCount) {
        return 'Goes wrong here when extension is activated';
    }