Application.Current in ElementHost is null

I use a WPF UserControl in my personal Libs. The Libs are included in my WPF and WindowsForms programs. Now my UserControl has to show a new (WPF) Window. At the new Window I want to set the Owner. I do it like this:

dialog.Owner = Application.Current.MainWindow;

This works fine, if i use the UserControl in a WPF program.

When I use the UserControl in my WindowsForms program (I set the UserControl in a ElementHost elementHost.Child = ...) is Application.Current null.

This is not good and my program throws an exception.

Why is Application.Current null?