Fix adding pages without associated window to wxTreebook

wxTreebook is supposed to allow not specifying any valid window for the
top-level pages, but this didn't work any longer, probably since the
changes of 02a92e23f3 (see #4379), as a
possibly null page was dereferenced without checking, resulting in a
crash.

Fix this by adding a missing check.

Also rename DoGetNonNullPage() to TryGetNonNullPage() to make it more
clear that this function can return null and add a unit test checking
that calling AddPage(NULL) really works (or at least doesn't crash).

See https://github.com/wxWidgets/wxWidgets/pull/921

src/generic/treebkg.cpp

@@ -542,7 +542,7 @@ void wxTreebook::MakeChangedEvent(wxBookCtrlEvent &event)
     event.SetEventType(wxEVT_TREEBOOK_PAGE_CHANGED);
 }
 
-wxWindow *wxTreebook::DoGetNonNullPage(size_t n)
+wxWindow *wxTreebook::TryGetNonNullPage(size_t n)
 {
     wxWindow* page = wxBookCtrlBase::GetPage(n);