Deleted imported models are kept in the Exportable list

[Brachi]

1. Import a model, e.g. pl0000.mod
2. Verify pl0000.mod appears in the exportable list
3. Delete the imported object and all its children
4. pl0000.mod remains in the exportable list, but when clicking export, the following error appears

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Albam error report
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Blender version: 4.2.1
Albam version: 0.4.0.dev11
Operating System: Linux-6.10.7-arch1-1-x86_64-with-glibc2.40
Error: ValueError: max() iterable argument is empty
Traceback:
  File "******/.config/blender/4.2/scripts/addons/albam/blender_ui/export_panel.py", line 167, in execute
    self._execute(context, item)
  File "******/.config/blender/4.2/scripts/addons/albam/blender_ui/export_panel.py", line 179, in _execute
    vfiles = export_function(item.bl_object)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "******/.config/blender/4.2/scripts/addons/albam/engines/mtfw/texture.py", line 783, in wrapper
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "******/.config/blender/4.2/scripts/addons/albam/engines/mtfw/material.py", line 999, in wrapper
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "******/.config/blender/4.2/scripts/addons/albam/engines/mtfw/mesh.py", line 763, in export_mod
    _serialize_top_level_mod(bl_meshes, src_mod, dst_mod)
  File "******/.config/blender/4.2/scripts/addons/albam/engines/mtfw/mesh.py", line 858, in _serialize_top_level_mod
    sphere = get_model_bounding_sphere(bl_meshes)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "******/.config/blender/4.2/scripts/addons/albam/lib/blender.py", line 159, in get_model_bounding_sphere
    radius = max(map(lambda vertex: get_dist(center, vertex), vertices))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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