Now is clear 
You need to pass the ref of your parent GUI (W_Menu) to the child one (StarMenu), my advice is to create a a W_Menu ref variable in the StarMenu called “Parent” and set it editable and expose as spawn. When you create StarMenu the creation node will require a W_Menu ref, put it in a reference to self. Then you can use the brand new “Parent” variable to cast.
EDIT: to be honest you don’t even need a cast, if you want to make your GUI a little more “”“universal”"" you can ask for a Widget ref instead of a W_Menu ref (as exposed variable). If you do this then you need a cast.