Description of problem:
Ansible tasks failed with "Failed to initialize: NoMethodError - undefined method `[]' for nil:NilClass" while updating the status of the stopped delegated tasks.

Steps to Reproduce:
1) Run an ansible script job with a command "sleep 60" on 1 or more clients.
2) Wait until the client's RunHostJob task is created by the main task (RunHostsJob) and it is in running state.
3) Then stop foreman-tasks for about 2 minutes until the remote task is finished.
4) Start foreman-tasks.
5) About 10 minutes later, the RunHostJob should failed with the above error.

How reproducible:
- Stopping the foreman-tasks in STEP 3 is to make the RunHostJob miss the API callback from the smart_proxy_dynflow_core when the remote task is stopped. Anyway that could cause the foreman tasks to miss the API callback should be able to reproduce the issue.
- If the Callback is missed, there is a Watcher in the main task (RunHostsJob) that will wakeup every 10 minutes to check the status of its sub tasks and notify them.
- While the sub stask (RunHostJob) updating its status, the task failed with the error above.


Actual results:
Task fail

Expected results:
foreman-tasks should update the status of the task and stop the task successfully without raising the above error