SoftReference for XWikiDocumentArchive improperly used

We use a SoftReference for keeping the document archive referenced from a document, for improving the memory consumption of the document cache, given the fact that the archive is something rarely used. From the definition of a SoftReference, the referenced object can be discarded by the garbage collector any time the system is low on free memory, which means that although the Reference object is not null, the referenced object may be null. It seems that we never check if this is happening, which sometimes causes a NullPointerException when using the document archive.