easyui-combobox 动态添加选项option

根据后台数据添加：

$("#themecombo").combobox({    
		    url:'<%=basePath%>analysis/getactivit.action',    
		    valueField:'id',
		    textField:'title',
		    editable:false
		});

然后想在最前面添加一个空选项：

 $(document).ready(
				function(){
					$.ajax({
					 url:'<%=basePath%>analysis/getactivit.action', 
					 type:'post',
					 success:function(data){
					 	var themecombo2 =[{ 'text':'请选择','id':''}];
							for(var i=0;i<data.length;i++){
							themecombo2.push({"text":data[i].title,"id":data[i].id});
							}
						$("#themecombo").combobox("loadData", themecombo2);
					 }
					});
		 });

<td class="label" id="lthemecombo">主题: </td>
<td class="content" id="lthemecomboi">
<input id="themecombo" class="easyui-combobox" name="prefecture" style="width:160px;" data-options="editable:false,valueField:'id', textField:'text'" />
</td>