Can't get Item Container from Backstage in Fluent Ribbon

Question

I cannot get item container from the ListBox in Backstage. Say, I have the following Backstage:

<!-- Backstage -->
<r:Ribbon.Menu>
  <r:Backstage x:Name="backStage">
    <r:BackstageTabControl>
      <r:BackstageTabItem Header="Columns">
        <Grid>
          <ListBox Grid.Row="1" Grid.Column="0" x:Name="lstColumns"/>
        </Grid>
      </r:BackstageTabItem>
    </r:BackstageTabControl>
  </r:Backstage>
</r:Ribbon.Menu>

I fill it up:

public Root()
{
  ContentRendered += delegate
  {
    var list = new List<int> { 1, 2, 3 };
    foreach (var index in list)
    {
      lstColumns.Items.Add(index);
    }
  };
}

Next, I want to retrieve the item container (in this case - ListBoxItem) from the first entry of ListBox:

private void OnGetProperties(object sender, RoutedEventArgs e)
{
  // Get first item container
  var container = lstColumns.ItemContainerGenerator.ContainerFromIndex(0);
  if (container is not null)
  {
    MessageBox.Show($"container = {container.GetType().FullName}");
  }
  else
  {
    MessageBox.Show("container is null");
  }
}

But container is always null. But! If I open Backstage and then hide it, I see the message:

container = System.Windows.Controls.ListBoxItem.

So, I decided to add code which opens Backstage before filling it up:

backStage.IsOpen = true;
var list = new List<int> { 1, 2, 3 };
foreach (var index in list)
{
  lstColumns.Items.Add(index);
}
backStage.IsOpen = false;

This works, but there's a flickering when you can barely see that Backstage is shown and hidden. This is not the perfect solution. So, how to get the item container?

P.S. Test project is here.

UPDATE (EXPLANATION)

The reason I need the item container is that I need to add set CheckBox state upon filling ListBox. This ListBoxis styled to contain CheckBoxes for items:

<Window.Resources>
  <Style x:Key="CheckBoxListStyle" TargetType="ListBox">
    <Setter Property="SelectionMode" Value="Multiple"/>
    <Setter Property="ItemContainerStyle">
      <Setter.Value>
        <Style TargetType="ListBoxItem">
          <Setter Property="Margin" Value="2"/>
          <Setter Property="Template">
            <Setter.Value>
              <ControlTemplate TargetType="ListBoxItem">
                <CheckBox Focusable="False"
                    IsChecked="{Binding Path=IsSelected,
                                        Mode=TwoWay,
                                        RelativeSource={RelativeSource TemplatedParent}}">
                  <ContentPresenter />
                </CheckBox>
              </ControlTemplate>
            </Setter.Value>
          </Setter>
        </Style>
      </Setter.Value>
    </Setter>
  </Style>
</Window.Resources>

So, when I add text in the loop above, the CheckBoxgets created. I, then, need to set the states of those checkboxes, which come from JSON. So, I need something like this:

var list = new List<int> { 1, 2, 3 };
var json = JsonNode.Parse("""
{
  "checked": true
}
""");
foreach (var index in list)
{
  CheckBox checkBox = null;
          
  var pos = lstColumns.Items.Add(index);
  var container = lstColumns.ItemContainerGenerator.ContainerFromIndex(pos);
  // Reach checkbox
  // ...
  // checkBox = ...
  // ...
  checkBox.IsChecked = json["checked"].GetValue<bool>();
}

And the problem is that container is always null. Also, it doesn't matter whether I use Loaded or ContentRendered event - in either case container is null.