This is a race condition, did not crash with -onethread. Attached the collected threaded callstacks, although they look misaligned.
Licensee has provided a suggested fix.
Download the attached repro (CustomDataRepro.zip) and open in 5.0 latest
Delete box number 3, crash should happen immediately.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-150941 in the post.