Predator and CL did not provide any possible suspects.
Using Code Search for the file, "Position.cpp" assigning to concern owner from GIT Blame.

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/5a61dd9f37830bdf66fdcc6fb9c38a9ccf607377

@yosin -- Could you please look into the issue, kindly re-assign if this is not related to your changes.
Thank You.

Lower to Pri-3 since real world usage of InsertHorizontalRule command is low.

ClusterFuzz has detected this issue as fixed in range 495506:495521.

Detailed report: https://clusterfuzz.com/testcase?key=5121641574301696

Fuzzer: bj_broddelwerk
Job Type: windows_asan_chrome_no_sandbox
Platform Id: windows

Crash Type: Null-dereference READ
Crash Address: 0x00000008
Crash State:
  blink::PositionTemplate<class blink::EditingAlgorithm<class blink::NodeTraversal
  blink::LastEditablePositionBeforePositionInRootAlgorithm<class blink::EditingAlg
  blink::LastEditablePositionBeforePositionInRoot
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=windows_asan_chrome_no_sandbox&range=467971:467992
Fixed: https://clusterfuzz.com/revisions?job=windows_asan_chrome_no_sandbox&range=495506:495521

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5121641574301696

See https://github.com/google/clusterfuzz-tools for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.

ClusterFuzz testcase 5121641574301696 is verified as fixed, so closing issue as verified.

If this is incorrect, please add ClusterFuzz-Wrong label and re-open the issue.