YQ-4847 fixed uncaught exception in external sources explain

[GrigoriyPA]

Changelog entry

Fixed uncaught exception in external sources explain

Changelog category

• Bugfix

Description for reviewers

Bugfix ticket: https://st.yandex-team.ru/YQ-4847

[github-actions]

🟢 2025-10-31 15:37:31 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-10-31 15:38:07 UTC Pre-commit check linux-x86_64-relwithdebinfo for 4ee4023 has started.
⚪ 2025-10-31 15:38:45 UTC Artifacts will be uploaded here
⚪ 2025-10-31 15:40:48 UTC ya make is running...
🟡 2025-10-31 17:12:45 UTC Some tests failed, follow the links below. Going to retry failed tests...

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
39115	36287	0	9	2791	28

⚪ 2025-10-31 17:12:59 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-10-31 17:22:53 UTC Tests successful.

Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
68 (only retried tests)	54	0	0	0	14

🟢 2025-10-31 17:22:56 UTC Build successful.
🟢 2025-10-31 17:23:13 UTC ydbd size 2.3 GiB changed* by +6.4 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 4af0266	merge: 4ee4023	diff	diff %
ydbd size	2 427 637 080 Bytes	2 427 643 672 Bytes	+6.4 KiB	+0.000%
ydbd stripped size	515 904 712 Bytes	515 906 120 Bytes	+1.4 KiB	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2025-10-31 15:39:02 UTC Pre-commit check linux-x86_64-release-asan for 4ee4023 has started.
⚪ 2025-10-31 15:39:21 UTC Artifacts will be uploaded here
⚪ 2025-10-31 15:40:42 UTC ya make is running...
🟡 2025-10-31 17:35:59 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
15523	15108	0	176	212	27

🟢 2025-10-31 17:36:08 UTC Build successful.
🟢 2025-10-31 17:36:30 UTC ydbd size 3.8 GiB changed* by +15.2 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 4af0266	merge: 4ee4023	diff	diff %
ydbd size	4 065 299 928 Bytes	4 065 315 512 Bytes	+15.2 KiB	+0.000%
ydbd stripped size	1 509 159 304 Bytes	1 509 164 872 Bytes	+5.4 KiB	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}