System.NotSupportedException: 'This type of Collection view does not support changes to its SourceCollection from a thread ...'

Question

Edit: My question is not answered by related threads because I want to avoid adding boilerplate every time I need to interact with an Observable Collection.

I do not need the application to be responsive while my loading operation plays out. What I need, really, is for one window (the loading popup) to animate and update even when the main window is frozen. I would have thought this was simple.

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I have a 3D scene containing a hierarchy of 3D objects. Some objects are serialised as Json, and I have a button on these objects called 'Export And Reimport', which serialises and reloads that object.

This process often involves rebuilding the 3D hierarchy to match the re-imported data. It can be time consuming.

I want to show a loading popup to assure the user that the program is still functioning. I've investigated a few ways of doing this, including BackgroundWorkers:

public BackgroundWorker _backgroundWorker = new BackgroundWorker();

public void ExportAndReimport()
{
    OpenLoadingWindow("Exporting and Reimporting ");
    
    _backgroundWorker.DoWork += new DoWorkEventHandler((s, e) =>
    {
        Export();
        Import();
    });
    
    _backgroundWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler((s, e) =>
    {
        CloseLoadingWindow();
    });
    
    _backgroundWorker.RunWorkerAsync();
}

and async operations:

public async Task ExportAndReimport()
{
    OpenLoadingWindow("Exporting and Reimporting ");
    
    await Task.Run(() =>
    {
        Export();
        Import();
        CloseLoadingWindow();
    });
}

In both cases, the problem is the same: my 3D objects contain many ObservableCollections (since the UI has to represent their properties, among other things). For example, a given object's Children property, which of course is re-populated on reloading the object. I'm not able to make changes to an ObservableCollection from a thread different to the one it was created on.

I just want to freeze input while the load is happening, and show an animated window (ideally, but not necessarily, with a progress bar). What's the best way to achieve this?