private function _getSectionSources(Element $element = null): array
    {
        $sources = [];
        $sections = Craft::$app->getSections()->getAllSections();
        $showSingles = false;

        foreach ($sections as $section) {
            if ($section->type === Section::TYPE_SINGLE) {
                $showSingles = true;
            } else if ($element) {
                // Does the section have URLs in the same site as the element we're editing?
                $sectionSiteSettings = $section->getSiteSettings();
                if (isset($sectionSiteSettings[$element->siteId]) && $sectionSiteSettings[$element->siteId]->hasUrls) {
                    $sources[] = 'section:'.$section->uid;
                }
            }
        }

        if ($showSingles) {
            array_unshift($sources, 'singles');
        }

        return $sources;
    }