Adds a window inside another window.
Adding windows inside another window creates "subwindows". These windows should not be added to application directly and are not accessible directly with any url. Addding windows implicitly sets their parents.
Only one level of subwindows are supported. Thus you can add windows inside such windows whose parent is null
.
@param window
@throws IllegalArgumentException if a window is added inside non-application level window.
@throws NullPointerException if the given
Window
is
null
.