After autooptimiser "NoneType" has no attribute 'toJson'

[Benvii]

I found out the following error on some panorama that weren't stitched in CBNB campaign :

INFO:opv_task.AutooptimiserTask.shell:Iteration: 247, error: 18.397890
INFO:opv_task.AutooptimiserTask.shell:Iteration: 248, error: 18.397890
INFO:opv_task.AutooptimiserTask.shell:Iteration: 249, error: 18.397785
INFO:opv_task.MakeallTask:End of task autooptimiser
INFO:opv_task.MakeallTask:Starting task stitchable
Traceback (most recent call last):
  File "/home/opv/venvs/opv/bin/opv-task", line 11, in <module>
    sys.exit(main())
  File "/home/opv/venvs/opv/lib/python3.5/site-packages/opv_tasks/__main__.py", line 57, in main
    lastTaskReturn = run(dir_manager_client, db_client, task_name, inputData)
  File "/home/opv/venvs/opv/lib/python3.5/site-packages/opv_tasks/__main__.py", line 73, in run
    return task.run(options=inputData)
  File "/home/opv/venvs/opv/lib/python3.5/site-packages/opv_tasks/task/task.py", line 94, in run
    taskOutput = self.runWithExceptions(options=options)
  File "/home/opv/venvs/opv/lib/python3.5/site-packages/opv_tasks/task/makealltask.py", line 58, in runWithExceptions
    self.logger.debug("TaskReturn : " + lastTaskReturn.toJSON())
AttributeError: 'NoneType' object has no attribute 'toJSON'

This might explain why we only have 70% of images stitched.