c# WPF Cant get Parent Window

Is the code that you posted in your constructor method?

The parent of a UserControl is always null in its constructor, so this.Parent is returning a null reference. Thus, calling Window.GetWindow(this.Parent) raises an ArgumentNullException because the dependency object that you specified has not been created yet.

To fix this, you need to place the code in the Initialized event handler. When this event is raised, you can be sure that the UserControl has been created.

Try Owner property, you have to assign it.

Sample:

public Activity ShowLookUp(Window owner)
{
     ActivityLookUp lookup = new ActivityLookUp();
     lookup.Owner = owner;
     lookup.ShowDialog();
}