title: yii2 plugins in dynamic-form wbraganca only work at first element
body: <p>I'm using some plugins (yii\widgets\MaskedInput and kartik\select2\Select2) in wbraganca dynamic-form.
  I notice these two plugins work only at first element.
  Please advice. Thanks before.
  my code like this</p>
  
  <pre><code>&lt;?= $form-&gt;field($detail, "[{$i}]item_id")-&gt;widget(Select2::classname(), [
      'data' =&gt; ArrayHelper::map(Item::find()-&gt;all(), 'id', 'name'),  
      'language' =&gt; 'en',
      'options' =&gt; ['placeholder' =&gt; 'Select Items ...', 'onchange' =&gt; 'getItemPrice($(this))'],
      'pluginOptions' =&gt; [
          'allowClear' =&gt; true
      ],
  ]) ?&gt;
  
  &lt;?= $form-&gt;field($detail, "[{$i}]qty")-&gt;widget(\yii\widgets\MaskedInput::className(),
  [
      'clientOptions' =&gt; [
      'alias' =&gt; 'numeric',
      'groupSeparator' =&gt; ',',
      'digits' =&gt; 0,
      'autoGroup' =&gt; true,
      'removeMaskOnSubmit' =&gt; true,
      'rightAlign' =&gt; false,                                  
      ],
      'options' =&gt; [
         'class' =&gt; 'form-control',
         'onchange' =&gt; 'calculateSubtotal($(this))',                                  
      ]                               
  ]) ?&gt;
  </code></pre>