I had a goal on my single player world. I started a world in 1.7.10, to get as many blacksmith villgers. You may ask what so special about them, well they have all diamonds trades. I been play on the emc server in 1.12 and haven't touch my single player world on current version, one day i wanted to work on the project so i switch back to 1.7.10 and try to enter the single player world. It crash every time. Now im 😢.
Hm... I don't have experience with such things. I do know that when Etho experienced game crashes, he usually tried opening the world in MCEdit to see if he could figure out and fix the cause. But sometimes you'll just need to load from a back-up. But I guess you haven't made any recently? :/
Good news everyone i know why the game crash, the render distance was up high. Bad news wish i knew that before switch it to 1.12. More good news i was'nt that far along, i can start again. ☺