Action update widget dynamicform not saved in Yii 2.0 database

Hello everyone, I can get a registration form, which has a main table po, and a secondary table po_item to save multiple records, but in the update action the form does not bring the records of the bank items follows the code

controller po

/**
     * Updates an existing Po model.
     * If update is successful, the browser will be redirected to the 'view' page.
     * @param integer $id
     * @return mixed
     */
    public function actionUpdate($id)
    {
        $model = $this->findModel($id);
        $modelsPoItem = [new PoItem];
        if ($model->load(Yii::$app->request->post()) && $model->save()) {
            return $this->redirect(['view', 'id' => $model->id]);
        } else {
            return $this->render('update', [
                'model' => $model,
                'modelsPoItem' => (empty($modelsPoItem)) ? [new PoItem] : $modelsPoItem
            ]);
        }
    }

_form Po

<div class="po-form">

    <?php $form = ActiveForm::begin(['id' => 'dynamic-form']); ?>

    <?= $form->field($model, 'po_no')->textInput(['maxlength' => 10]) ?>
    <?= $form->field($model, 'description')->textarea(['rows' => 6]) ?>

    <div class="row">
        <div class="panel panel-default">
        <div class="panel-heading"><h4><i class="glyphicon glyphicon-envelope"></i> Po Items</h4></div>
        <div class="panel-body">
             <?php DynamicFormWidget::begin([
                'widgetContainer' => 'dynamicform_wrapper', // required: only alphanumeric characters plus "_" [A-Za-z0-9_]
                'widgetBody' => '.container-items', // required: css class selector
                'widgetItem' => '.item', // required: css class
                'limit' => 10, // the maximum times, an element can be cloned (default 999)
                'min' => 1, // 0 or 1 (default 1)
                'insertButton' => '.add-item', // css class
                'deleteButton' => '.remove-item', // css class
                'model' => $modelsPoItem[0],
                'formId' => 'dynamic-form',
                'formFields' => [
                    'po_item_no',
                    'quantity',
                ],
            ]); ?>
            <div class="container-items"><!-- widgetContainer -->
            <?php foreach ($modelsPoItem as $i => $modelPoItem): ?>

                <div class="item panel panel-default"><!-- widgetBody -->
                    <div class="panel-heading">
                        <h3 class="panel-title pull-left">Po Item</h3>
                        <div class="pull-right">
                            <button type="button" class="add-item btn btn-success btn-xs"><i class="glyphicon glyphicon-plus"></i></button>
                            <button type="button" class="remove-item btn btn-danger btn-xs"><i class="glyphicon glyphicon-minus"></i></button>
                        </div>
                        <div class="clearfix"></div>
                    </div>
                    <div class="panel-body">
                        <?php
                            // necessary for update action.
                            if (! $modelPoItem->isNewRecord) {
                                echo Html::activeHiddenInput($modelPoItem, "[{$i}]id");
                            }
                        ?>
                        <div class="row">
                            <div class="col-sm-6">

                                <?= $form->field($modelPoItem, "[{$i}]po_item_no")->textInput(['maxlength' => 128]) ?>
                            </div>
                            <div class="col-sm-6">
                                <?= $form->field($modelPoItem, "[{$i}]quantity")->textInput(['maxlength' => 128]) ?>
                            </div>
                        </div><!-- .row -->
                    </div>
                </div>
            <?php endforeach; ?>
            </div>
            <?php DynamicFormWidget::end(); ?>
        </div>
        </div>
    </div>
    <div class="form-group">
        <?= Html::submitButton($model->isNewRecord ? 'Create' : 'Update', ['class' => $model->isNewRecord ? 'btn btn-success' : 'btn btn-primary']) ?>
    </div>

    <?php ActiveForm::end(); ?>

</div>