The map editor does not open a previously created map.

I had the same problem, I struggled with it for a long time. Sometimes I create a really huge map, but the save file is only 1KB, and after that, neither the editor nor the game can open it. I searched for an answer here but didn't find it. Eventually, I remembered that in the description of some maps (on this site), it says that they are mods and should be deleted after playing. I removed all the maps from the Maps folder, and then the editor started working.