Fix TypeError in truncate_serialized_error when message is None

[Krishnachaitanyakc]

Summary

Fixes #33662

• Bug: truncate_serialized_error calls len(error_info.message) without guarding against None. When deserializing older event log entries, the message field of SerializableErrorInfo can be None (the @record decorator has checked=False, so runtime type enforcement is disabled). This causes a TypeError: object of type 'NoneType' has no len() crash when viewing failed runs in the Dagster UI via the RunStepStatsQuery GraphQL query.

• Fix: Fall back to an empty string (error_info.message or "") before computing the length and slicing, consistent with the defensive pattern already used for cls_name on the same function (line 271: if error_info.cls_name and len(...)).

• Test: Added a regression test test_truncate_serialized_error_none_message that constructs a SerializableErrorInfo with message=None and verifies truncate_serialized_error handles it without raising.

Test plan

• Added test_truncate_serialized_error_none_message regression test
• Existing test_truncate_serialized_error continues to pass
• Manual verification: the fix matches exactly what the issue reporter validated in their own environment

[greptile-apps]

Greptile Summary

This PR fixes a TypeError crash in truncate_serialized_error when error_info.message is None, which can happen when deserializing older event log entries stored with the @record(checked=False) decorator. The fix adds a defensive or "" fallback before computing len() and slicing, consistent with the existing guard already applied to cls_name in the same function.

Key changes:

• error.py: Replaces len(error_info.message) with msg = error_info.message or "" / len(msg), and uses msg for slicing, ensuring None messages are handled gracefully and the original None is preserved when no truncation is needed.
• test_errors.py: Adds a targeted regression test test_truncate_serialized_error_none_message that constructs a SerializableErrorInfo with message=None, invokes truncate_serialized_error, and asserts no exception is raised and the None value is preserved.

Confidence Score: 5/5

• This PR is safe to merge — it is a minimal, targeted bug fix with a regression test and no side effects on the normal code path.
• The change is a one-line defensive guard that exactly mirrors the existing cls_name guard in the same function, produces no behavioral change for non-None messages, and is covered by a new regression test. The risk surface is essentially zero.
• No files require special attention.

Important Files Changed

Filename	Overview
python_modules/dagster/dagster/_utils/error.py	Adds a None guard for error_info.message before calling len(), fixing a TypeError crash when deserializing older event log entries with a None message field.
python_modules/dagster/dagster_tests/utils_tests/test_errors.py	Adds test_truncate_serialized_error_none_message regression test that verifies truncate_serialized_error no longer raises TypeError when message=None, and confirms the None value is preserved in the result.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[truncate_serialized_error called] --> B{error_info.message is None?}
    B -- Yes --> C["msg = '' (empty string)"]
    B -- No --> D["msg = error_info.message"]
    C --> E["msg_len = len(msg)  →  0"]
    D --> F["msg_len = len(msg)"]
    E --> G{"msg_len > field_size_limit?"}
    F --> G
    G -- No --> H[message unchanged / None preserved]
    G -- Yes --> I["error_info._replace(message=msg[:limit] + ' (TRUNCATED)')"]

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