There is no need to set the owner of the application main window.
The correct way to set the owner of a window in XAML is as follows
<object>
<object.Owner>
<Window .../>
</object.Owner>
</object>
alternatively you can set it from code behind like this
// Create a window and make this window its owner
Window ownedWindow = new Window();
ownedWindow.Owner = this;
ownedWindow.Show();
The way you have done WPF tries to convert the string "Auto_Calibrator_v0._05.MainWindow" to window object which is not possible unless you have a type converter.