WPF InvalidOperationException Getting MainWindow Reference - wpf

Question

I have MainWindow, which contains an instance of DemoUI (a UserControl).
From within a class instance called DemoModule, I have a reference to DemoUI which I call _demoUI.
When I try to get a reference to the MainWindow from within DemoModule using
var parentWindow = Window.GetWindow(_demoUI);
I get this InvalidOperationException:
The calling thread cannot access this object because a different thread owns it.
Ultimately, I want to be able to update the MainWindow's progress bar's value using it's Dispatcher as follows:
var progressBar = parentWindow.FindName("ProgressBar") as ProgressBar;
progressBar.Dispatcher.Invoke(DispatcherPriority.Normal,
new DispatcherOperationCallback(o => {
progressBar.Value = Progress = args.Current;
return null;
}), null);
Update 1
public void OnProgressChanged(object sender, ProgressChangedEventArgs args)
{
Progress = Convert.ToInt32(args.Current * 100);
var progressBar = Application.Current.MainWindow.FindName("ProgressBar") as ProgressBar;
if (progressBar != null)
progressBar.Value = Progress;
}

Answer 1

From the code you include here it doesn't seem like you need to get the progressBar reference in order to update the ProgressBar on your _demoUI? Is there another ProgressBar on the MainWindow? Regardless, you can use the _demoUI reference to get to the dispatcher.
_demoUI.Dispatcher.Invoke(DispatcherPriority.Normal,
new DispatcherOperationCallback(o => {
_demoUI.ProgressBar.Value = Progress = args.Current;
return null;
}), null);
Or
_demoUI.Dispatcher.BeginInvoke(DispatcherPriority.Background,
new DispatcherOperationCallback(o => {
var window = Window.GetWindow(_demoUI);
//do what you need to with the window here.
}), null);