Don't crash when creating wxStaticBox in wxGTK.

Don't crash when creating wxStaticBox in wxGTK.

Since r74585, wxWindow::SetCanFocus() is called from wxStaticBoxBase ctor,
i.e. before the real window is created, and this results in a crash in wxGTK
where SetCanFocus() needs a valid widget.

Fix this simply by doing nothing in SetCanFocus() if the widget is not created
yet as this should result in the same behaviour as before r74585. This doesn't
seem like the right thing to do, however, and we should probably remove this
call from wxStaticBoxBase ctor and do it in wxStaticBox implementation itself
instead.

Closes #15358.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74594 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775