Issue #3509064 by lukasz.tyc: Undefined array key "class" in...

Issue #3509064 by lukasz.tyc: Undefined array key "class" in template_preprocess_toc_tree() in toc_api.module

Closes #3509064

toc_api.module

@@ -120,7 +120,7 @@ function template_preprocess_toc_tree(&$variables) {
   $variables['options'] = $toc->getOptions();
 
   // Add default classes.
-  $variables['attributes']['class'] = (array) $variables['attributes']['class'];
+  $variables['attributes']['class'] = isset($variables['attributes']['class']) ? (array) $variables['attributes']['class'] : [];
   $variables['attributes']['class'][] = 'toc';
   $variables['attributes']['class'][] = 'toc-tree';