yii.validation is not defined error in yii2 - yii2

Question

I am opening a ActiveForm in modal window in yii2.
This error comes when I am opening a ActiveForm in Modal window . my index.php is
<?php
use yii\helpers\Html;
use yii\bootstrap\Modal;
$this->title = 'Roles';
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="role-index">
<h1><?= Html::encode($this->title) ?></h1>
<p>
<?= Html::a('Create Role', '#', [
'class' => 'btn btn-success',
'id' => 'create-role-model',
'data-toggle' => 'modal',
'data-target' => '#activity-create-modal',
]) ?>
</p>
</div>
<?php
Modal::begin([
'id' => 'activity-create-modal',
'header' => '<h2>Hello world</h2>',
'footer' => Html::button('Close', ['class' => 'btn btn-default', 'data-dismiss' => 'modal'])
. PHP_EOL . Html::button('Add', ['class' => 'btn btn-primary btn-modal-save']),
]);
$model= new \frontend\models\Role();
echo $this->renderAjax('_form',['model' => $model]);
Modal::end();
?>
and my _form.php is
<?php
use yii\helpers\Html;
use yii\bootstrap\ActiveForm;
/* #var $this yii\web\View */
/* #var $model frontend\models\Role */
/* #var $form yii\widgets\ActiveForm */
?>
<?php $form = ActiveForm::begin([
'id' => 'create-ro',
'enableClientValidation' => true,
]); ?>
<?= $form->field($model, 'id')->textInput(['maxlength' => true]) ?>
<?= $form->field($model, 'created_on')->textInput() ?>
<?= $form->field($model, 'updated_on')->textInput() ?>
<div class="form-group">
<?= Html::submitButton($model->isNewRecord ? 'Create' : 'Update', ['class' => $model->isNewRecord ? 'btn btn-success' : 'btn btn-primary']) ?>
</div>
<?php ActiveForm::end(); ?>
However when I place ActiveForm without render in index.php Yii Validation is working.Am I doing it an wrong manner . What have to be done so that modal validation start

Answer 1

It's echo $this->render('_form',['model' => $model]); you need, not $this->renderAjax().
The latter actually terminates the entire page and returns it without a layout around it (so including any javascript includes and so on).
Since you are still in another view that is definitely not what you need. So just use render(). It's a bit confusing because Controller::render() actually does close the page (and also adds javascripts), but in case of the View (Which $this is in that context) it does what renderPartial does for the controller.