i am trying to create a dataStore to populate a combobox, everything seems to work fine, but my combobox does not show up at the place where i want, actually it does not show up anywhere.

i am trying something like this.

//this is the content pane where i want the combobox

      var tab = new dijit.layout.ContentPane({
      title: "tab",
      style:"width:100%; height:500px",
      content: "Here goes the comobox",



   });

  tab_container.addChild(tab);

  // this is the combobox and store created

   Types = function() {

dojo.xhrGet
({url:"json/types.json",
handleAs: "json",
load: function(types, details) {
        PropertyMap.TypeStore = new dojo.data.ItemFileReadStore({
            data : {
                identifier: 'id',
                label: 'display',
                items : types
            }
        });
    },
 error: function(error_msg, details) {
        content.handleError(error_msg, details);
    }});
 };

 XXX.createPlaceTypeStore = function(types) {



var PlaceTypeStore = new dojo.data.ItemFileReadStore({
    data:{
        identifier:"id",
        label:"display",
        items:types
        }
    });

    var combo_box = new dijit.form.ComboBox({
name : "options",
placeHolder:"options place",
value:"",
store: PlaceTypeStore,
searchAttr:"display"
 });

 combo_box.placeAt("tab");



   // i am getting this error
  refNode is null
 [Break On This Error]  

   if(has("dojo-combo-api")){