When you call .combobox({...}) to create the component, the data items is not loaded immediately, it may take a little while to load the data from remote server. Before loading data successfully, you call 'select' method to select a un-existing record. So the 'onSelect' event fires but the record does not exist. To solve this issue, please 'select' a record after loading data successfully.
$('#xxx').combobox({
onselect:function(record){
console.log(record.value);
},
onLoadSuccess:function(){
$(this).combobox('select','somevalue')
}
})