NPE in XWikiDocument.CopyDocument() introduced by the patch of XWIKI-4283

This is a mistery for me, but it seems BaseObject vectors in XWikiDocument may contains null object. This is checked almost anywhere in the code, this is poor design but since it is, we need to support it. And I know, this is hard, I have made this mistake myself.

Here is a patch for version 1.9.x.