A PopupWindow (RadialPopupMenu is a subclass of that) is not designed to be added as a child window - so it won't behave correctly.
The only reason that you can add it is that a run time check would be a bit expensive.
To handle inputs events for a popup you can override handleEventPopup().
The RPM is designed to be opened from a mouse click which then transfers the (potential) drag gesture to the RPM which allows to do single stroke menus. It should close as soon as you release the mouse button. You can try the GameUIDemo