The NPE occurred when for an arbitrary segment no parent documents exist for a has_parent filter/query and no child documents exist for a has_child filter/query.

Closes #2297

The NPE occurred when for an arbitrary segment no parent documents exist for a has_parent filter/query and no child documents exist for a has_child filter/query.

Closes elastic#2297

These members are default initialized on contruction and then set by the
init() method.  It's possible that another thread accessing the object
after init() is called could still see the null/0 values, depending on how
the compiler optimizes the code.

These members are default initialized on contruction and then set by the
init() method.  It's possible that another thread accessing the object
after init() is called could still see the null/0 values, depending on how
the compiler optimizes the code.

* es/6.x: (155 commits)
  Make persistent tasks work. Made persistent tasks executors pluggable.
  Removed ClientHelper dependency from PersistentTasksService.
  Added AllocatedPersistentTask#waitForPersistentTaskStatus(...) that delegates to PersistentTasksService#waitForPersistentTaskStatus(...)
  Add adding ability to associate an ID with tasks.
  Remove InternalClient and InternalSecurityClient (#3054)
  Make the persistent task status available to PersistentTasksExecutor.nodeOperation(...) method
  Refactor/to x content fragments2 (#2329)
  Make AllocatedPersistentTask members volatile (#2297)
  Moves more classes over to ToXContentObject/Fragment (#2283)
  Adapt to upstream changes made to AbstractStreamableXContentTestCase (#2117)
  Move tribe to a module (#2088)
  Persistent Tasks: remove unused isCurrentStatus method (#2076)
  Call initialising constructor of BaseTasksRequest (#1771)
  Always Accumulate Transport Exceptions (#1619)
  Pass down the provided timeout.
  Fix static / version based BWC tests (#1456)
  Don't call ClusterService.state() in a ClusterStateUpdateTask
  Separate publishing from applying cluster states
  Persistent tasks: require allocation id on task completion (#1107)
  Fixes compile errors in Eclipse due to generics
  ...