ext combox添加空值项

listeners:{expand:initComboBox}

function initComboBox() {
		      if (!this.initialized) {
		          this.initialized = true;
		          var TYPE = Ext.data.Record.create([{name:this.valueField}, {name:this.displayField}]);
		          var r = eval('new TYPE({' + this.valueField + ':"",' + this.displayField + ':"---"});');
		          this.store.insert(0, r);
		      }
		}