code: protected function renderDataCellContent($model, $key, $index)
      {
          if ($this->content !== null) {
              return call_user_func($this->content, $model, $key, $index, $this);
          }
  
          return $this->grid->emptyCell;
      }
docs: Renders the data cell content.
  @param mixed $model the data model
  @param mixed $key the key associated with the data model
  @param int $index the zero-based index of the data model among the models array returned by [[GridView::dataProvider]].
  @return string the rendering result