After first deleting a class document and then a document containing an object of that class, the stored XML of the object won't include a class definition. Later, when reading the document from the trash, the re-parsed object won't have a classname set, which causes a NPE in this.xObjects.get(object.getXClassReference()) in com.xpn.xwiki.doc.XWikiDocument.setXObject(int, BaseObject). The whole Deleted Documents page becomes unusable.
A proper fix of the BaseObject toXML and fromXML methods would be needed, but it's hard to fix without breaking a lot of backwards compatibility. Something to work on later, for the moment a quick fix for the object's classname should fix the problem at hand.