Predator and CL could not provide any possible suspects.
Using Code Search for the file, "replaceselectioncommand.cpp" assigning to the concern owner.

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/442257beaaf7120042ef6483c85fed3e6703618a

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

ClusterFuzz has detected this issue as fixed in range 499699:499749.

Detailed report: https://clusterfuzz.com/testcase?key=4733298768347136

Fuzzer: bj_broddelwerk
Job Type: windows_syzyasan_chrome
Platform Id: windows

Crash Type: Null-dereference
Crash Address: 0x0000000b
Crash State:
  blink::ReplaceSelectionCommand::DoApply
  blink::CompositeEditCommand::ApplyCommandToComposite
  blink::CompositeEditCommand::MoveParagraphs
  
Memory Tool: SYZYASAN

Regressed: https://clusterfuzz.com/revisions?job=windows_syzyasan_chrome&range=498677:498753
Fixed: https://clusterfuzz.com/revisions?job=windows_syzyasan_chrome&range=499699:499749

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=4733298768347136

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 4733298768347136 is verified as fixed, so closing issue as verified.

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