Object Reference error on loading custom map

Hi there,

I've created a custom My Pet Dungeon map and I can play it just fine, it seems. However, whenever I save the game and reload the save, I always get this Object Reference error.

It does not matter when I save the game. It can be after an hour of playtime, but it can also be right after starting the map, the error always appears when loading it.

I have unlocked the ballista, the two multi-block hero doors, the stone knight and that hero carpet for building.

The map also contains 4 quartz blocks with gold in them and two toy boxes, both with customized minions (heroes and monsters). Just in case any of these "special" changes could potentially cause such problems.

Here's the output log:

output_log.txt

And here are the save files (tell me if you need my custom map as well):

kaiserlicher spielplatz_20190429114456.meta
kaiserlicher spielplatz_20190429114456

Hope you can help!