NavigationViewItem.ContainerFromMenuItem returns null

[rcohn]

Hello -

I am building a WinUI3 unpackaged desktop app that uses a NavigationView. One of the features of the app is that I must force an expand or collapse of a NavigationView item under programmatic control. To do that, the only way I know of is to determine the selected item and then get the NavigationViewItem via ContainerFromMenuItem so that I can set the NavigationView's Expanded property. The NavigationView is mostly used through a collection of menu items built as an observable collection. For example, I don't add items directly to the NavigationView; I add through the item collection.

Here is a small code example that shows what I'm trying to do to expand a NavigationViewItem:

In this example, ContainerFromMenuItem returns a null so I can't complete the operation to expand the item.
However, if I call ExpandMenuItem from a NavigationView event handler (such as the _Expanding handler, which gets called when the user clicks on an item), then ContainerFromMenuItem returns a valid, non-null object.

In my example, I always get a valid non-null CNavViewItem object in the call to navView.SelectedItem. That part works fine.

But how do I ensure that ContainerFromMenuItem always returns a non-null object?

I realize what I've shown here is very patchy (and I apologize for the image, but I couldn't make the edit markup show a presentable format for the code), but hopefully there's enough for somebody to recognize what I could be doing wrong - or if in fact there is a bug in ContainerFromMenuItem. Please let me know if more information would help.

Many thanks for any assistance that can be provided for this perplexing problem.
Robert