Scaffold

Closes #3462525

src/ClassyLayout.php

@@ -66,9 +66,10 @@ class ClassyLayout extends LayoutDefault implements PluginFormInterface {
       if (empty($class_definition['options']) || !is_array($class_definition['options'])) {
         throw new \Exception('The "options" key is required for layout class definitions.');
       }
-      if ($class_definition['multiple'])  {
+      if ($class_definition['multiple'] ?? FALSE) {
         $definition_default = $class_definition['default'] ?? [];
-      } else {
+      }
+      else {
         $definition_default = $class_definition['default'] ?? NULL;
       }
       $form['classes'][$key] = [