NPE in PictureTilesAdapter

There is a NPE inside the PictureTilesAdapterImpl that make the big picture preview not usable.

There are actually 3 problems :

• tiles hashmap is not threadsafe : but this is not the main issue
• cleanup() method of adapter set the tiles hashmap to null and noone reset it (main cause of the NPE)
• the cleanup() method is called for bad reasons because the date comparison is broken (comparing a live date with date comming from the DB that has been rounded)

The second problem is probably here for a long time but was not visible until the third problem arise.