Merge pull request #425 from MaximeOh/master

Fix the class attribute when the field has add-ons

Author: ADmad

src/View/Widget/InputGroupTrait.php

@@ -73,6 +73,7 @@ protected function _withInputGroup(array $data, ContextInterface $context): stri
                 $errorClass &&
                 $context->hasError($data['fieldName'])
             ) {
+                $attrs['class'] = (array)$attrs['class'];
                 $attrs['class'][] = $errorClass;
             }