This looks similar to   Issue 795842 , hence merging into it.
Please undo it, if it not the case

Doesn't look like the same as  issue 795842 .

Lower to P3 due to low usage of InsertHorizontalRule command.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1a9a57460a35f8cc7d1897aea94ab7c290504554

commit 1a9a57460a35f8cc7d1897aea94ab7c290504554
Author: tanvir.rizvi <tanvir.rizvi@samsung.com>
Date: Tue Jan 02 06:31:29 2018

Fix for crash with ReplaceSelectionCommand

On applying ReplaceSelecionCommand with
no visible selection reaches NOTREACHED.
We should abort the editing state instead.

Bug:  796840 
Change-Id: I0291b7f70602a9e0ef20501392882912451de9f8
Reviewed-on: https://chromium-review.googlesource.com/846321
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Tanvir Rizvi <tanvir.rizvi@samsung.com>
Cr-Commit-Position: refs/heads/master@{#526441}
[modify] https://crrev.com/1a9a57460a35f8cc7d1897aea94ab7c290504554/third_party/WebKit/Source/core/editing/commands/ReplaceSelectionCommand.cpp
[modify] https://crrev.com/1a9a57460a35f8cc7d1897aea94ab7c290504554/third_party/WebKit/Source/core/editing/commands/ReplaceSelectionCommandTest.cpp

ClusterFuzz has detected this issue as fixed in range 526440:526441.

Detailed report: https://clusterfuzz.com/testcase?key=6256936575303680

Fuzzer: bj_broddelwerk
Job Type: linux_cfi_chrome
Platform Id: linux

Crash Type: Null-dereference READ
Crash Address: 0x000000000020
Crash State:
  chrome
  blink::Node::GetDocument
  blink::HasEditableStyle
  
Sanitizer: cfi (CFI)

Regressed: https://clusterfuzz.com/revisions?job=linux_cfi_chrome&range=523197:523221
Fixed: https://clusterfuzz.com/revisions?job=linux_cfi_chrome&range=526440:526441

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=6256936575303680

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.