RadioButton Functionality in Salesforce Lightning

RadioButtons.cmp
----------------
<aura:component >
     <ui:inputRadio text="Demo1" label="Demo1"  change="{!c.onClick}"/><br/>
    <ui:inputRadio  text="Demo2" label="Demo2"  change="{!c.onClick}"/><br/>
   
</aura:component>

RadioButtonsController.js
-------------------------

({
    onClick: function(component, event) {
        var selected = event.getSource().get("v.text");
        alert(selected);
    }

})

Lightning ApplicationException
--------------------------------

<aura:application extends="force:slds" >
    <c:RadioButtons/>
</aura:application>