MenuItems need to be presented in a Menu...
<popup name="popup1" placementtarget="{binding elementName= btn}" placement="bottom">
<menu>
<menuitem header="File">
<menuitem header="Open" />
<menuitem header="Save" />
</menuitem>
<menuitem header="Edit" />
<menuitem header="View" />
<menuitem header="window" />
</menu>
<popup>
</popup></popup>