This is a pointer to a MUI object and defines the
contents of your window. Usually, this root object
will be of class MUIC_Group since you surely want to
have more than one gadget.
The root object is treated as child of a window
and will be disposed when the window is disposed.
Note that windows can only have one child.
Although you may create a window without root object,
you have to set one before the window is openend!