Lucene plugin index updater randomly broken

That's what breaks the Lucene page (before XPLUCENE-50) with a NullPointer exception.

Here is the state:

• the variable LucenePlugin#indexUpdater is null at some point
• here is the only ways to have this variable to null:
  • lucene plugin init failed: impossible to have that without error log (but nobody reported having any error log when getting this issue)
  • lucene plugin has been loaded and the constructor called but #init() methods has not been called
  • flushCache has been called and the init called by LucenePlugin#flushCache failed: again that should produce error log for most use cases (i will add some proper log in plugin manager to make sure to cover any Exception which should be done anyway)