I was playing in a hardcore world and after dying to a creeper, the "Spectate World" and "Delete World" options popped up. I clicked "Delete World". It returned me to the main menu where the rotating background screen had disappeared and turned into a whitish, gray screen behind the menu buttons. Opening the singleplayer game button and returning to the menu didn't clear the issue. The hardcore world still appeared in the worlds screen and I got curious and attempted to open it. The game then froze with a dark gray screen and had to be killed in the Task Manager.
How to replicate:
Create a hardcore world, die and click the "delete world" option.
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duplicates
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Thank you for your report!
However, this issue is a Duplicate of MC-148865.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Just recreated in release 1.14.2. Still an issue. Updated Java environment to 8 update 211.