jQuery Grid Plugin - jqGrid

If there is a grid with group headers then the sortableColumns => update() function fails as the group headers are th elements, but they don't have ids.

See the highlighted part where it fails.

Javascript error: Cannot read property 'id' of undefined.

This seems to be happening because the group headers are the th elements, while this code below assumes that each th is a column. Simply adding an if checking for whether the id exist eliminates the error, but then those columns which are grouped together are completely ignored.

jquery.jqGrid.src.js:

$.jgrid.extend({
  sortableColumns : function (tblrow)
  {
  ...
  "update": function(event, ui) {
     var p = $(ui.item).parent(),
     th = $(">th", p),
     colModel = ts.p.colModel,
     cmMap = {}, tid= ts.p.id+"_";
     $.each(colModel, function(i) { cmMap[this.name]=i; });
     var permutation = [];
     th.each(function() {
          var id = $(">div", this).get(0).id.replace(/^jqgh_/, "").replace(tid,"");
          if (cmMap.hasOwnProperty(id)) {
               permutation.push(cmMap[id]);
          }
     });

     $(ts).jqGrid("remapColumns",permutation, true, true);
     if ($.isFunction(ts.p.sortable.update)) {
          ts.p.sortable.update(permutation);
     }
     setTimeout(function(){ts.p.disableClick=false;}, 50);
  }

....