code: public function renderPreviousExceptions($exception)
      {
          if (($previous = $exception->getPrevious()) !== null) {
              return $this->renderFile($this->previousExceptionView, ['exception' => $previous]);
          } else {
              return '';
          }
      }
docs: Renders the previous exception stack for a given Exception.
  @param \Exception $exception the exception whose precursors should be rendered.
  @return string HTML content of the rendered previous exceptions.
  Empty string if there are none.