Internal Server Error when running characterisation in mock environment

[axelboc]

1. Sync samples with ISPyB
2. Mount first sample
3. Run characterisation
4. Characterisation appears to be running in UI but never completes (and cannot be stopped) because server has errored:

2024-07-11 10:55:33,341 |MX3.HWR|DEBUG  | [2024-Jul-11 10:55] 127.0.0.1 GET http /mxcube/api/v0.1/queue/queue_state? 5xx INTERNAL SERVER ERROR
Traceback (most recent call last):
  File "/home/bocciare/tools/micromamba/envs/mxcubeweb/lib/python3.10/site-packages/flask/app.py", line 880, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/bocciare/tools/micromamba/envs/mxcubeweb/lib/python3.10/site-packages/flask/app.py", line 865, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
  File "/home/bocciare/tools/micromamba/envs/mxcubeweb/lib/python3.10/site-packages/flask_security/decorators.py", line 432, in decorated_view
    return current_app.ensure_sync(fn)(*args, **kwargs)
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/routes/queue.py", line 130, in queue_get_state
    resp = jsonify(app.queue.get_queue_state())
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 260, in get_queue_state
    queue = self.queue_to_dict(include_lims_data=True)
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 175, in queue_to_dict
    self.queue_to_dict_rec(node, include_lims_data),
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 734, in queue_to_dict_rec
    result.append(self._handle_sample(node, include_lims_data))
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 695, in _handle_sample
    "tasks": self.queue_to_dict_rec(node, include_lims_data),
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 762, in queue_to_dict_rec
    result.extend(self.queue_to_dict_rec(node, include_lims_data))
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 740, in queue_to_dict_rec
    result.append(self._handle_char(sample_node, node, include_lims_data))
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 596, in _handle_char
    originID, task = self._handle_diffraction_plan(node, sample_node)
  File "/home/bocciare/workspace/mxcubeweb/mxcubeweb/core/components/queue.py", line 625, in _handle_diffraction_plan
    for col in collections:
TypeError: 'NoneType' object is not iterable