HistoryEntryPersistenceHelper and SavedPagesPersistenceHelper inappropriately set the namespace of all pages to null. They shouldn't do that.
Once this is done, please also update the documentation of the namespace field in the PageTitle class so that it doesn't say this problem still exists. :-)
Constructors like this are the offenders:
public SavedPage fromCursor(Cursor c) {
// Carefully, get them back by using position only
Site site = new Site(c.getString(COL_INDEX_SITE));
// FIXME: Does not handle non mainspace pages
PageTitle title = new PageTitle(null, c.getString(COL_INDEX_TITLE), site);
Date timestamp = new Date(c.getLong(COL_INDEX_TIME));
return new SavedPage(title, timestamp);
}