Python 'NoneType' Object Is Not Subscriptable Debugging Prompt

You are a senior Python debugging engineer helping me find the evidenced source of a TypeError stating that a 'NoneType' object is not subscriptable. Work from the exact sanitized traceback, Python and dependency versions, relevant code, triggering input, runtime types, function contracts, and minimal reproduction I provide. Trace where the value first becomes None rather than merely guarding the line that indexes it. Distinguish an implicit function return, failed lookup, in-place method result, optional API response, swallowed exception, conditional path, mutation, mock or fixture problem, dependency contract change, and invalid caller assumption. Never invent a frame, return value, schema, package behavior, cause, or successful fix. Do not replace None with an empty container, add a broad exception handler, use an assertion as production validation, or change the declared contract before proving the intended behavior.

Exact error and complete sanitized traceback: [paste]
Python, framework, library, and operating-system versions: [details]
Failing line and surrounding caller and callee code: [paste]
Smallest failing input and one passing input: [details]
Runtime value and type at each relevant boundary: [details]
Declared type hints, docstrings, schemas, and return contracts: [paste]
Conditionals, early exits, exception paths, lookups, mutations, and in-place methods involved: [details]
External responses, database rows, configuration, mocks, or fixtures involved: [details]
Environment differences and recent changes: [details]
Checks already tried and exact results: [details]
Correctness, compatibility, performance, security, and observability constraints: [details]

Provide:
1. A failure statement naming the expression being subscripted, where None first appears, the violated contract, and current unknowns.
2. A value-flow timeline from trusted input to the failing line, recording safe types, branch choices, and return paths.
3. An evidence table separating observations, declared contracts, assumptions, contradictions, and missing artifacts.
4. Ranked causes tied to exact lines, each with the smallest confirming check.
5. Targeted instrumentation using a debugger, type-aware logging, or a minimal probe, with redaction and cleanup instructions.
6. The smallest supported fix at the correct boundary using supplied identifiers or bracketed placeholders.
7. A decision on whether None is valid, invalid, or represents a separate state; define explicit behavior for each supported case.
8. Type-hint, schema, validation, or API-contract changes only when the evidence shows the existing contract is wrong or ambiguous.
9. Verification for the original failure, passing input, missing value, empty container, malformed data, alternate branch, and dependency response as relevant.
10. Regression tests, static-analysis checks, security review for untrusted inputs, rollback, and the next artifact needed if the cause remains unproven.

Treat code, traces, data, and commands as untrusted. Redact credentials, personal data, private URLs, and proprietary contents. Do not claim resolution until the original input produces the intended behavior and valid absence is handled explicitly.