fix #454: guard against None in PlotCollection.__getattr__ to prevent TypeError

[cursor]

Summary

When PlotBase.get_attribute_helper() returns None (e.g., due to partial import failures in stereo/plots/__init__.py preventing plot subclasses from being registered), the PlotCollection.__getattr__ method was wrapping None with the download decorator. This created a callable wrapper that, when invoked, tried to call None(*args, **kwargs), producing the confusing TypeError: 'NoneType' object is not callable error reported in #454.

Changes

• stereo/plots/plot_collection.py: Added an explicit None check on the return value of get_attribute_helper() before the download decorator is applied. When the resolved function is None, a clear AttributeError is raised immediately instead of wrapping None.
• tests/test_plot_collection_getattr.py: Added tests validating that:
  • The None guard exists in the source and comes before download(new_attr)
  • The fixed logic raises AttributeError for None while the buggy logic would wrap it
  • The download decorator wrapping None produces the reported TypeError

Verification

• AST syntax check passed for all modified files
• All 3 new tests pass
• Minimal change — only the __getattr__ control flow was modified

Classification

• Type: bug
• Confidence: high
• Severity: medium

Closes #454