Sort by distance don't work

fix #4260: sort by distance doesn't work
I can revert to the old style implementation, where `cacheComparator` is set to `null` when `DistanceComparator` is used.

But as side effect:
* sort order cannot be reversed when sorted by `DistanceComparator` (see CacheListActivity.onCreateOptionsMenu() )
* a list with only Events cannot be sorted by `DistanceComparator` ( CacheListAdapter.getCacheComparator() )

For the `NullPointerException` I could instead initialize it to empty ArrayList...

What's your advise?