Hi Jacques,
I was able to reproduce this crash here, and it seems to have been introduced by CL 40129784. Your fix seems appropriate, although for performance you could also have replaced the operator (which is checked for key existence) by a non-checked accessor to avoid a double lookup.
I have created an internal bug report for this issue. You should be able to track it here once the engine devs mark it as public: UE-296335.
Best regards,
Vitor