how does itemcontainergenerator.containerfromitem work grouped list?

i have listbox until recently displaying boring list items. i means mylist.itemcontainergenerator.conainerfromitem(thing) collect listboxitem hosting "thing" list.

this week i've run-down listbox rather collectionviewsource binds the equipment organization enabled. equipment within listbox grouped underneath good headers.

however, given doing this, itemcontainergenerator.containerfromitem stopped operative - advantage zero even equipment i know listbox. heck - containerfromindex(0) returning zero even listbox populated items!

how i collect listboxitem listbox that's displaying grouped items?

edit: here's xaml code-behind trimmed-down example. raises nullreferenceexception since containerfromindex(1) returning zero even nonetheless there 4 equipment list.

xaml:

<window x:class="wpfapplication1.window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:scm="clr-namespace:system.componentmodel;assembly=windowsbase"
    title="window1">

    <window.resources>
        <xmldataprovider x:key="mytasks" xpath="tasks/task">
            <x:xdata>
                <tasks xmlns="">
                    <task name="groceries" type="home"/>
                    <task name="cleaning" type="home"/>
                    <task name="coding" type="work"/>
                    <task name="meetings" type="work"/>
                </tasks>
            </x:xdata>
        </xmldataprovider>

        <collectionviewsource x:key="mysortedtasks" source="{staticresource mytasks}">
            <collectionviewsource.sortdescriptions>
                <scm:sortdescription propertyname="@type" />
                <scm:sortdescription propertyname="@name" />
            </collectionviewsource.sortdescriptions>

            <collectionviewsource.groupdescriptions>
                <propertygroupdescription propertyname="@type" />
            </collectionviewsource.groupdescriptions>
        </collectionviewsource>
    </window.resources>

    <listbox 
        x:name="listbox1" 
        itemssource="{binding source={staticresource mysortedtasks}}" 
        displaymemberpath="@name"
        >
        <listbox.groupstyle>
            <groupstyle>
                <groupstyle.headertemplate>
                    <datatemplate>
                        <textblock text="{binding name}"/>
                    </datatemplate>
                </groupstyle.headertemplate>
            </groupstyle>
        </listbox.groupstyle>
    </listbox>
</window>

cs:

public window1()
{
    initializecomponent();
    listbox1.itemcontainergenerator.statuschanged += itemcontainergenerator_statuschanged;
}

void itemcontainergenerator_statuschanged(object sender, eventargs e)
{
    (listbox1.itemcontainergenerator.status == system.windows.controls.primitives.generatorstatus.containersgenerated)
    {
        listbox1.itemcontainergenerator.statuschanged -= itemcontainergenerator_statuschanged;

        var i = listbox1.itemcontainergenerator.containerfromindex(1) listboxitem;

        // name keyboard-focus second item
        i.isselected = true;
        i.focus();
    }
}