wxCompositeWindow::GetCompositeWindowParts() becomes simpler to implement in the derived classes with optionally shown elements if NULL windows are allowed (and ignored) in the list returned by it. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68910 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775