code: def _find_element(self, selector, selector_type, check_displayed=False):
          
          if selector_type == 'css':
              elements = self.driver.find_elements_by_css_selector(selector)
          elif selector_type == 'xpath':
              elements = self.driver.find_elements_by_xpath(selector)
          elif selector_type == 'id':
              elements = self.driver.find_elements_by_css_selector('#' + selector)
          else:
              raise RuntimeError(
                  "Unknown selector_type: %s for selector: %s"
                  % (selector_type, selector)
              )
          for element in elements:
              if check_displayed:
                  if not element.is_displayed() or sum(element.size.values()) <= 0:
                      continue
              return element
docs: Return first matching displayed element of non-zero size
          or None if nothing found