Android - Find id of item in SubMenu

I use Navigation activities for moving from one Activity to another. And I want to add submenu to menu of NavigationView< but can't find way to find id og item in submenu.

    <menu>    
<item android:title="Communicate">
    <menu>
        <item android:id="@+id/about_us" android:icon="@android:drawable/ic_menu_share"
            android:title="About us" />
    </menu>
</item></menu>

 

@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    switch (id) {
        case R.id.nav_order: {
            break;
        }
        case R.id.nav_search: {
            break;
        }
    }

    return true;
}

I want to add: case R.id.about_us. But Android Studio can't find this id. How can i make it and what i do wrong?