wxFrame::m_taskBarButton needs to be initialized in its Init(), not Create(),
which is not necessarily used by the derived classes such as wxMDIParentFrame,
resulting in a guaranteed crash when deleting an uninitialized pointer later
in wxFrame dtor.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77688 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775