rem: treeCellChildren[i].setAttribute("flex", "1"); treecell.appendChild(treeCellChildren[i]); 
add:  if (treeCellChildren[i]) { treeCellChildren[i].setAttribute("flex", "1"); treecell.appendChild(treeCellChildren[i]); } 
context: function constructRow(treeCellChildren){    var treeitem = document.createElement("treeitem");    var row = document.createElement("treerow");    for (var i = 0; i<treeCellChildren.length; i++) {      var treecell = document.createElement("treecell");      treecell.setAttribute("allowevents", "true");      treeCellChildren[i].setAttribute("flex", "1");      treecell.appendChild(treeCellChildren[i]);      row.appendChild(treecell);    }    treeitem.appendChild(row);    return treeitem;}
meta: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/607652df165dcb9c47bac9ae15fafb56b3da496b/FilterEditor.js/clean/mailnews/base/search/resources/content/FilterEditor.js