NPE while handling CheckpointNotice in KafkaSupervisor

[jihoonson]

The error occurs at the below check in the CheckpointNotice class.

      Preconditions.checkNotNull(
          sequenceTaskGroup.get(sequenceName),
          "WTH?! cannot find task group for this sequence [%s], sequencesTaskGroup map [%s], taskGroups [%s]",
          sequenceName,
          sequenceTaskGroup,
          taskGroups
      );

This happens in the below scenario.

1. One of tasks in a taskGroup has succeeded, and the supervisor stopped all tasks in that group.
2. One of tasks sent a checkpoint request to the supervisor before stopping.
3. The supervisor received the checkpoint request after tidying up all information of the taskGroup.