My brother jumped into the voice of the doom in my Hardcore Creative map.
I didn't press the ΄΄**Delete This World**΄΄ button yet.
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After dieing, choose to go into spectator mode instead of deleting the world (for obvious reasons). Then open the world to LAN and set `cheats enabled` to `ON`.

Run the command `/gamemode survival`.  

Save and exit.

You should now be able to load your world again and you will be in survival mode, cheats will be disabled and your next death will be permanent (unless you follow these steps again).

*Note: I tested this in 1.14.4, it may not work in previous versions*
Don't click the "Delete World" button instead close out Minecraft and write down the seed before you make your world.