Update frontend_routing.module

Closes #3440109

frontend_routing.module

@@ -10,6 +10,7 @@
  */
 function frontend_routing_form_node_form_alter(&$form, &$form_state) {
   $nodes = \Drupal::state()->get('frontend_routing.settings');
+  if (is_array($nodes) && !empty($nodes)) {
   $nodes = array_combine($nodes, $nodes);
   $node = $form_state->getFormObject()->getEntity();
   if (!$node->isNew() && in_array($node->id(), $nodes)) {
@@ -17,4 +18,5 @@ function frontend_routing_form_node_form_alter(&$form, &$form_state) {
     $form['path_replace']['#weight'] = $form['path']['#weight'];
     $form['path_replace']['#markup'] = '<strong>' . t('This node is managed by Frontend Routing.') . '</strong>';
   }
+  }
 }