Issue #2840579 by ckng, Mirnaxvb: 'is-active' class is not in array

src/Widget/WidgetPluginBase.php

@@ -164,7 +164,7 @@ abstract class WidgetPluginBase extends PluginBase implements WidgetPluginInterf
     }
 
     if ($result->isActive()) {
-      $items['#attributes'] = ['class' => 'is-active'];
+      $items['#attributes'] = ['class' => ['is-active']];
     }
 
     $items['#wrapper_attributes'] = ['class' => $classes];