Appears to be deleting a model then complaining it can't find it!

[DrMark666]

I haven't been using this for long; however, I have processed a few files successfully, and I also have a couple (of STL files) that lock pycam up, I can see errors relating to not finding a model in collections - which I can see it deleting a little earlier.
This run, I left for some hours and got a report which says to report it, so here goes.
The STL iin use is https://www.thingiverse.com/thing:2519083 if that helps - appears OK & passes checks for slic3r OK - I've rotated it to flat before use, so it's a save from slic3r. & not the original!

I did see a vaguely similar ticket in 2017, but it's not the same.

An unexpected exception occurred: please send the text below to the developers of PyCAM. Thanks a lot!
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/usr/lib/python3.8/multiprocessing/pool.py", line 125, in worker
result = (True, func(*args, **kwds))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/PushCutter.py", line 36, in _process_one_line
points = get_free_paths_triangles(models, cutter, p1, p2)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/init.py", line 81, in get_free_paths_triangles
(cl1, d1, cp1) = cutter.intersect(backward, t, start=p1)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Cutters/ToroidalCutter.py", line 271, in intersect
(cl_t, d_t, cp_t) = self.intersect_circle_triangle(direction, triangle, start=start)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Cutters/BaseCutter.py", line 129, in intersect_circle_triangle
(cl, ccp, cp, d) = self.intersect_circle_plane(direction, triangle, start=start)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Cutters/ToroidalCutter.py", line 208, in intersect_circle_plane
cl = psub(cp, psub(ccp, start))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Geometry/PointUtils.py", line 92, in psub
return (a[0] - b[0], a[1] - b[1], a[2] - b[2])
TypeError: 'NoneType' object is not subscriptable
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Plugins/Tasks.py", line 261, in generate_toolpath
if new_toolpath.get_toolpath() is None:
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 292, in wrapped
return self.get_cached(inst, args, kwargs, calc_function)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 347, in get_cached
cache_item = CacheItem(time.time(), calc_function(inst, *args, **kwargs))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 237, in inner_function
result = func(self, *args, **kwargs)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 1465, in get_toolpath
toolpath = task.generate_toolpath()
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 292, in wrapped
return self.get_cached(inst, args, kwargs, calc_function)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 347, in get_cached
cache_item = CacheItem(time.time(), calc_function(inst, *args, **kwargs))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 237, in inner_function
result = func(self, *args, **kwargs)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 1350, in generate_toolpath
moves = path_generator.generate_toolpath(
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/PushCutter.py", line 77, in generate_toolpath
result = self.generate_toolpath_slice(cutter, models, layer_grid, draw_callback,
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/PushCutter.py", line 127, in generate_toolpath_slice
for points in run_in_parallel(_process_one_line, args, callback=progress_counter.update):
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Utils/threading.py", line 647, in run_in_parallel_local
for result in imap_func(func, args):
File "/usr/lib/python3.8/multiprocessing/pool.py", line 868, in next
raise value
TypeError: 'NoneType' object is not subscriptable

Storing a state revision (16/20)
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/usr/lib/python3.8/multiprocessing/pool.py", line 125, in worker
result = (True, func(*args, **kwds))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/PushCutter.py", line 36, in _process_one_line
points = get_free_paths_triangles(models, cutter, p1, p2)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/init.py", line 81, in get_free_paths_triangles
(cl1, d1, cp1) = cutter.intersect(backward, t, start=p1)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Cutters/ToroidalCutter.py", line 271, in intersect
(cl_t, d_t, cp_t) = self.intersect_circle_triangle(direction, triangle, start=start)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Cutters/BaseCutter.py", line 129, in intersect_circle_triangle
(cl, ccp, cp, d) = self.intersect_circle_plane(direction, triangle, start=start)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Cutters/ToroidalCutter.py", line 208, in intersect_circle_plane
cl = psub(cp, psub(ccp, start))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Geometry/PointUtils.py", line 92, in psub
return (a[0] - b[0], a[1] - b[1], a[2] - b[2])
TypeError: 'NoneType' object is not subscriptable
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Plugins/OpenGLWindow.py", line 730, in paint
self.camera.position_camera()
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Plugins/OpenGLWindow.py", line 884, in position_camera
low, high = self._get_low_high_dims()
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Plugins/OpenGLWindow.py", line 768, in _get_low_high_dims
self.core.call_chain("get_draw_dimension", low, high)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Utils/events.py", line 266, in call_chain
data.func(*args, **kwargs)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Plugins/OpenGLViewToolpath.py", line 55, in get_draw_dimension
tp = toolpath_dict.get_toolpath()
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 292, in wrapped
return self.get_cached(inst, args, kwargs, calc_function)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 347, in get_cached
cache_item = CacheItem(time.time(), calc_function(inst, *args, **kwargs))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 237, in inner_function
result = func(self, *args, **kwargs)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 1465, in get_toolpath
toolpath = task.generate_toolpath()
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 292, in wrapped
return self.get_cached(inst, args, kwargs, calc_function)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 347, in get_cached
cache_item = CacheItem(time.time(), calc_function(inst, *args, **kwargs))
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 237, in inner_function
result = func(self, *args, **kwargs)
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/workspace/data_models.py", line 1350, in generate_toolpath
moves = path_generator.generate_toolpath(
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/PushCutter.py", line 77, in generate_toolpath
result = self.generate_toolpath_slice(cutter, models, layer_grid, draw_callback,
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/PathGenerators/PushCutter.py", line 127, in generate_toolpath_slice
for points in run_in_parallel(_process_one_line, args, callback=progress_counter.update):
File "/usr/local/lib/python3.8/dist-packages/pycam-0.7.0_pre0.606.g55e3129f-py3.8.egg/pycam/Utils/threading.py", line 647, in run_in_parallel_local
for result in imap_func(func, args):
File "/usr/lib/python3.8/multiprocessing/pool.py", line 868, in next
raise value
TypeError: 'NoneType' object is not subscriptable