[2.0.55] Crash in save menu (ServerSynchronizer::pruneSentHeartbeats)

[JohnAlekseyev]

Hey there. I cannot reproduce this crash, but since this is the first time in perhaps 7 years that I had Factorio crash, I decided to post the log anyway just in case it can help.

Basically: I was playing a session with a friend, saving as we were about to finish. Then we actually played for 2 or 3 more minutes and I wanted to save again, and as I was about to change the savegame name but before actually saving, the game crashed. I don't know for certain whether I had already typed some characters or only just clicked the text field.
I later saw that an autosave also took place between us saving the first time (worked fine) and attempting to re-save.
Only very few mods were used, which ran fine for us since starting this run in November. (Listed in log)

No noteworthy progress was lost and the previous save appears to run fine, but since I've never ever seen Factorio crash before (good job making an extremely stable game!), I am little bit concerned anyway.

I have attached the log, as well as the last manual save and the autosave shortly after.
The game's installation folder is named 2.0.23, but that's because I installed the standalone installation of that version and since then updated via the game's automatic updates, the actual version is 2.0.55.