code: public function getAssetManager()
      {
          if (!is_object($this->_assetManager)) {
              $options = $this->_assetManager;
              if (!isset($options['class'])) {
                  $options['class'] = 'yii\\web\\AssetManager';
              }
              if (!isset($options['basePath'])) {
                  throw new Exception("Please specify 'basePath' for the 'assetManager' option.");
              }
              if (!isset($options['baseUrl'])) {
                  throw new Exception("Please specify 'baseUrl' for the 'assetManager' option.");
              }
  
              if (!isset($options['forceCopy'])) {
                  $options['forceCopy'] = true;
              }
  
              $this->_assetManager = Yii::createObject($options);
          }
  
          return $this->_assetManager;
      }
docs: Returns the asset manager instance.
  @throws \yii\console\Exception on invalid configuration.
  @return \yii\web\AssetManager asset manager instance.