code: private static void handleDelayedSlotFill(DelayedSlotFillTask task) {
      Key slotKey = task.getSlotKey();
      Slot slot = querySlotOrAbandonTask(slotKey, true);
      Key rootJobKey = task.getRootJobKey();
      UpdateSpec updateSpec = new UpdateSpec(rootJobKey);
      slot.fill(null);
      updateSpec.getNonTransactionalGroup().includeSlot(slot);
      backEnd.save(updateSpec, task.getQueueSettings());
      // re-reading Slot (in handleSlotFilled) is needed (to capture slot fill after this one)
      handleSlotFilled(new HandleSlotFilledTask(slotKey, task.getQueueSettings()));
    }
docs: Fills the slot with null value and calls handleSlotFilled