Starting a new Editor Extensions map: https://imgur.com/a/xRQjcDh
Freeplay seems unaffected

I can't reproduce with just Milestones and Editor Extensions. Can you post your complete list of mods?

Here's a new editor save with all my enabled mods (except Milestones):
https://drive.google.com/file/d/1Ceuc0gHFClS4HSOkLcfNT5IVIo1OP-9A/view?usp=sharing
Enabling the mod after creating the editor save did not trigger the crash. Only enabling it with all the other mods before hitting Play did.

Thanks for the save file, I reproduced with 3 mods enabled: Milestones, Editor Extensions, and Spider sentinel.
Now to figure out why...

For some reason the Spider sentinel mod has a tech for a mini-spider that just straight up gives you items when you research it. A missed leftover debug?

tech.name = "spidersentinel_tinyspider"
tech.effects=
    {
        { type = "unlock-recipe", recipe = "spidersentinel_tinyspider" },
        { type="give-item", item="roboport", count="5"},
        { type="give-item", item="logistic-robot", count="10"},
        { type="give-item", item="construction-robot", count="10"},
        { type="give-item", item="logistic-chest-passive-provider", count="5"}
    }

Because EE researches all techs before the start of the game, these items end up being "produced" before the start of the game, and then Milestones sees the milestone as complete but can't find when it happened.
I'll just have to hardcode it so that not finding the time turns into 0:00. I'll release the fix tomorrow after the locale updates.

Fixed in 1.3.1.