SG-45127 Fix AttributeError in ShotgunHierarchyModel when first path is not loaded

[stevelittlefish]

Problem

ShotgunHierarchyModel.async_item_from_paths raised AttributeError: 'NoneType' object has no attribute 'index' whenever the first path in the list was not yet loaded in the model. This happens, for example, when an app such as the Loader auto-navigates to an entity while the model's root children have not been fetched yet. Reference: SG-45127.

Root cause

Two related defects in shotgun_hierarchy_model.py:

1. async_item_from_paths sliced paths[idx - 1:] to hand _NodeRefresher an already-loaded ancestor at index 0 plus the awaited node at index 1. For idx == 0 this wrapped to paths[-1:] — the deepest, never-loaded path. The loop implicitly assumed the first path is always already loaded.

2. _NodeRefresher.__init__ then called model.item_from_path(path_to_refresh[0]).index() unconditionally. Fed the deepest path by defect 1, item_from_path() returned None and .index() raised.

Fix

When idx == 0, refresh from the model root (self._path) — the first path's real parent, which always exists — instead of wrapping to the deepest path.

Guard _NodeRefresher.__init__ against item_from_path() returning None, aborting the refresh gracefully rather than crashing.

Testing

Added test_async_item_from_paths_first_path_not_loaded, which drives the idx == 0 branch and asserts it no longer raises.

Verified against the real model code that the previous logic crashed with the reported AttributeError while the updated logic refreshes from the root instead (nothing-loaded state aborts gracefully via the guard; root-loaded state issues a fetchMore on the root).

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.32%. Comparing base (35f3736) to head (150f9b0).

Files with missing lines	Patch %	Lines
python/shotgun_model/shotgun_hierarchy_model.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   61.63%   62.32%   +0.69%     
==========================================
  Files          50       50              
  Lines        3412     3419       +7     
==========================================
+ Hits         2103     2131      +28     
+ Misses       1309     1288      -21     

Flag	Coverage Δ
Linux	62.24% <72.72%> (+0.72%)	⬆️
Python-3.10	62.34% <72.72%> (+0.72%)	⬆️
Python-3.11	62.31% <72.72%> (+0.72%)	⬆️
Python-3.13	62.34% <72.72%> (+0.72%)	⬆️
Python-3.9	62.34% <72.72%> (+0.69%)	⬆️
Windows	62.26% <72.72%> (+0.69%)	⬆️
macOS	62.26% <72.72%> (+0.72%)	⬆️

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[carlos-villavicencio-adsk]

LGTM