docs: function (domElement) {
                  var popups = this.popups;
                  for (var i = 0, l = popups.length; i < l; i++) {
                      var curPopup = popups[i];
                      if (curPopup.domElement == domElement) {
                          return curPopup;
                      }
                  }
                  return null;
              }