WPF closing child- closes parent-window

i have the pretty same sample as mentioned here. Fast concluded: MainWindow closes when the last childwindow is closed.

My Problem: I couldn't solve my problems with the described solutions. I can't produce a program where it als takes place. Only in one of my bigger progs. Maybe someone has an idea or knows any further steps.

Thanks for reading - Thomas

As requested here's a bit of code: This is the part in the MainWindow:

bool editAfterSearch = false;
Movie selectedMovie = (Movie)this.listView.SelectedItem;
Movie backup = (Movie)selectedMovie.Clone();            

if (new OnlineSearchWindow().EditMovieViaOnlineSearch(ref selectedMovie, out editAfterSearch))
{
    this.coverFlow.Update(selectedMovie);
}

And that's the part of the ChildWindow:

public bool EditMovieViaOnlineSearch(ref Movie preset, out bool editAfter)
{
    this.exitWithOk = false;
    this.editMovieAfterSearch = false;

    this.tbx_SearchTerm.Text = preset.Title;
    this.linkedMovie = preset;

    this.ShowDialog();

    editAfter = editMovieAfterSearch;

    if (this.exitWithOk)
    {
        this.linkedMovie.CloneOnlineInformation(ref preset);
        preset.Bitmap = this.linkedMovie.Bitmap;

        return true;
    }
    else
    {
        return false;
    }
}

Try playing with the ShutDownMode property of your App.xaml.cs. The 3 values are OnMainWindowClose, OnLastWindowClose, and OnExplicitShutdown, and the default is OnLastWindowClose

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        this.ShutdownMode = System.Windows.ShutdownMode.OnMainWindowClose;
    }
}