Add defensive checks for Slim SEO plugin compatibility

Add null checks in UI element output() methods to prevent errors when
Slim SEO processes shortcodes before WP Ultimo is fully initialized.

This fixes the 'Call to a member function get_active_site_url() on null'
error by returning empty string instead of attempting to access null objects.

Changes:
- Add defensive checks in 5 UI elements output() methods
- Return empty string when site/membership objects are null
- Prevents fatal errors during Slim SEO's shortcode processing

Files modified:
- inc/ui/class-account-summary-element.php
- inc/ui/class-billing-info-element.php
- inc/ui/class-current-site-element.php
- inc/ui/class-invoices-element.php
- inc/ui/class-limits-element.php

Author: vuckro

inc/ui/class-account-summary-element.php

@@ -294,6 +294,11 @@ public function setup_preview(): void {
 	 */
 	public function output($atts, $content = null) {
 
+		// Defensive check - setup() may have been called but site can still be null
+		if ( ! $this->site) {
+			return '';
+		}
+
 		$atts = array_merge($atts, $this->atts);
 
 		$atts['site'] = $this->site;

inc/ui/class-billing-info-element.php

@@ -272,6 +272,11 @@ public function setup_preview(): void {
 	 */
 	public function output($atts, $content = null) {
 
+		// Defensive check - setup() may have been called but membership can still be null
+		if ( ! $this->membership) {
+			return '';
+		}
+
 		$atts['membership'] = $this->membership;
 
 		$atts['billing_address'] = $this->membership->get_billing_address();

inc/ui/class-current-site-element.php

@@ -352,6 +352,11 @@ public function register_scripts(): void {
 	 */
 	public function output($atts, $content = null) {
 
+		// Defensive check - setup() may have been called but site can still be null
+		if ( ! $this->site) {
+			return '';
+		}
+
 		$actions = [
 			'visit_site' => [
 				'label'        => __('Visit Site', 'multisite-ultimate'),

inc/ui/class-invoices-element.php

@@ -274,6 +274,11 @@ public function setup_preview(): void {
 	 */
 	public function output($atts, $content = null) {
 
+		// Defensive check - setup() may have been called but membership can still be null
+		if ( ! $this->membership) {
+			return '';
+		}
+
 		$atts['membership'] = $this->membership;
 
 		return wu_get_template_contents('dashboard-widgets/invoices', $atts);

inc/ui/class-limits-element.php

@@ -240,6 +240,11 @@ public function setup_preview(): void {
 	 */
 	public function output($atts, $content = null) {
 
+		// Defensive check - setup() may have been called but site can still be null
+		if ( ! $this->site) {
+			return '';
+		}
+
 		$post_types = get_post_types(
 			[
 				'public' => true,