Ensure a valid page is initially selected in the widgets sample

Select first page if an invalid one was restored which may happen if the
build of the library has changed (e.g. some previously disabled
wxUSE_XXX became enabled etc).

Closes https://github.com/wxWidgets/wxWidgets/pull/1235

samples/widgets/widgets.cpp

@@ -639,7 +639,7 @@ void WidgetsFrame::InitBook()
 #if USE_TREEBOOK
     // for treebook page #0 is empty parent page only so select the first page
     // with some contents
-    if ( !pageSet )
+    if ( !pageSet || !m_book->GetCurrentPage() )
         m_book->SetSelection(1);
 
     // but ensure that the top of the tree is shown nevertheless
@@ -648,7 +648,7 @@ void WidgetsFrame::InitBook()
     wxTreeItemIdValue cookie;
     tree->EnsureVisible(tree->GetFirstChild(tree->GetRootItem(), cookie));
 #else
-    if ( !pageSet )
+    if ( !pageSet || !m_book->GetCurrentPage() )
     {
         // for other books set selection twice to force connected event handler
         // to force lazy creation of initial visible content