add:  if ( typeof t == "undefined" ) { return; }; 
context: proto._onmousedown = function(e){  var t = e.getManagerTarget();    if (t instanceof QxImage) {    t = t.getParent();  };  if (t instanceof QxListItem) {    if( !t.isEnabled() ) {      return;    };    t = t.getParent();  };    switch(t)  {    case this._textfield:      return;        case this._atom:    case this._button:    case this._buttonimage:      this._togglePopup();      this._button.setBorder(QxBorder.presets.inset);      break;          case this._list:      this._list._onmousedown(e);            // don't close on click and move to the scrollbars      if (e.getTarget() != this._list) {        this._closePopup();      };            break;          // Mshtml click on scrollbar    case this._popup:      break;        default:      var sel = this.getSelected();            this._manager.deselectAll();            if (sel) {        this._manager.setSelectedItem(sel);      };          this._closePopup();       break;  };};
meta: 1005	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/1005/177eead4a0727d043b09ff468acc34ab6a84070d/QxComboBox.js/clean/qooxdoo/source/script/widgets/QxComboBox.js