code: protected function renderDropdown()
      {
          $lines = [];
          $lines[] = $this->encodeText ? Html::encode($this->text) : $this->text;
          if ($this->icon && is_string($this->icon)) {
              $lines[] = $this->icon;
          }
          $lines[] = $this->renderItems($this->items, $this->options, $this->displaySearchInput);
          return Html::tag('div', implode("\n", $lines), $this->options);
      }
docs: Renders the dropdown
  
  @return string the generated dropdown module
  @throws InvalidConfigException