backport of r60458 from trunk: don't put NULL pointers in the GdkWindowArray

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_9_0_BRANCH@60467 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/radiobox.cpp

@@ -608,7 +608,9 @@ GdkWindow *wxRadioBox::GTKGetWindow(wxArrayGdkWindows& windows) const
     {
         GtkWidget *button = GTK_WIDGET( node->GetData()->button );
 
-        windows.push_back(button->window);
+        // don't put NULL pointers in the 'windows' array!
+        if (button->window)
+            windows.push_back(button->window);
 
         node = node->GetNext();
     }