Put the summary of the bug you're having here:
I created a temporary new world to give the end world a try, defeated the boss and decided to go back my normal sp save world. Before I did this I deleted the 'temporary world' that i created, but it wouldn't delete.
So closed and opened MC and tried again, clicked on the temporary world and clicked delete, however, it deleted my other world, the one which I have spent hundreds, if not thousands of hours playing on.

What I expected to happen was...:
Describe what you thought should happen here:
The 'temporary world' I clicked on should've deleted, not the other world I had already there.

What actually happened was...:
Describe what happened here:
The other world, which was my main world in single-player in creative mode, deleted. After several attempts of re-opening and re-closing MC the temporary world eventually deleted.

Steps to Reproduce:
1. Create a world: world name 'New World', game mode-creative, difficulty- normal, generate structures- on, world type- default, allow cheats- on, bonus chest- off.
2. Open new world and wait for it to load completely, immediately go into inventory and use 'Eye of Ender', use this to find the nearest stronghold.
3. Once you have dug down to the portal, (I personally explored stronghold for about 10 minutes before going through portal), and place Eye of Ender on each 'End Portal' block, go into portal.
4. Equip yourself with diamond sword and bow and arrow.
5. Destroy all the Ender Crystals on each Obsidian Pillar.
6. Defeat the dragon with both bow and arrow and diamond sword (This took some time but it was worth the fight!) During the fight I used potion of poison with the bow, don't even know if it worked but tried it out.
7. Once the dragon is defeated go through the portal, watch the credits and wait till you are back in the world.
8. Exit the game but not minecraft. Click on the world you were playing in and click delete. *It might be an idea to have another world already created as this is it deleted this one instead of the 'temporary world.'
9. Watch the world you spent hundreds of hours on disappear, and the one you created to be temporary stay there, which refused to be deleted after several attempts of deleting it...