title: Kartik's yii2-grid: Setting unknown property: kartik\grid\GridView::panel
body: <p>For the life of me, I'm unable to sort the following error out when using Kartik's yii2-grid:</p>
  
  <p><strong>Setting unknown property: kartik\grid\GridView::panel</strong></p>
  
  <p>My code is:</p>
  
  <pre class="lang-php prettyprint-override"><code>use yii\helpers\Html;
  use kartik\grid\GridView;
  
  [...]
  
      &lt;?= GridView::widget([
          'dataProvider' =&gt; $dataProvider,
          'filterModel' =&gt; $searchModel,
          'hover' =&gt; true,
          'persistResize' =&gt; true,
          'pager' =&gt; [
               'firstPageLabel' =&gt; Yii::t('app', 'Inicio'),
               'lastPageLabel' =&gt; Yii::t('app', 'Final'),
               'maxButtonCount' =&gt; Yii::$app-&gt;params['numBotones'],
            ],
          'panel ' =&gt; [
              'heading'=&gt;'&lt;h1&gt;'.Html::encode(mb_strtoupper($this-&gt;title)).'&lt;/h1&gt;', 
          ],
          'columns' =&gt; [
               ['class' =&gt; 'kartik\grid\SerialColumn'],
  
              [...]
  </code></pre>
  
  <p>However, <code>hover</code> and <code>persistResize</code> do work.</p>
  
  <p>Also my <code>composer.json</code> contains:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>    "minimum-stability": "stable",
      "require": {
          "php": "&gt;=5.4.0",
          "yiisoft/yii2": "*",
          "yiisoft/yii2-bootstrap": "*",
          "yiisoft/yii2-swiftmailer": "*",
          "yiisoft/yii2-jui": "*",
          "kartik-v/yii2-grid": "@dev",
          "kartik-v/yii2-mpdf": "@dev",
          "kartik-v/yii2-widgets": "*",
          "2amigos/yii2-tinymce-widget" : "*",
          "sjaakp/yii2-illustrated-behavior": "*",
          "dektrium/yii2-user": "0.9.*@dev",
          "dektrium/yii2-rbac": "1.0.0-alpha@dev"
      },</code></pre>
  </div>
  </div>
  </p>
  
  <p>Any ideas?</p>